Onion & Mustard Tart with Brioche and Mustard Sauce!

We’re going to make a lovely onion & mustard tart dish! It’s the fourth Christmas episode and I love this one! It’s a gluten free buckwheat tart with caramelized onion, a mustard sauce, sweet & sour pearl onions, brioche and toasted & puffed mustard seeds.

The gluten free buckwheat tart shell

125 g	buckwheat flour
65 g	cold cubes of butter
25 g	beaten egg
3 g	salt
15 g	water

Start by pouring the buckwheat flour on your worktop and make a well in the middle. Fill it with the cold cubes of butter, the beaten egg, the salt and the water. Now bring all the ingredients together and knead it into a nice ball of dough. Then cover it and let it rest in your fridge for at least 1 hour. After the hour dust your worktop with some buckwheat flour and start rolling the dough. It needs to be around 3 millimeters thin. If the dough starts sticking use some more buckwheat flour. Once you’re happy with the thickness cut a small part in long strips and use those to cover the inside of a tart ring. This is quite a difficult shape, but you can use any shape you like. Then use the back of your knife or a pallet knife to trim the top and press the dough into the baking ring once more. You want to be absolutely sure it’s tight, otherwise it might collapse while baking it. Now press the ring into the remaining part of the dough sheet and remove any excess dough. Then place it on a perforated silicon sheet and let it cool down in your fridge for at least 30 minutes. After that bake it at 160 degrees Celsius for about 15 to 20 minutes. If you don’t have a perforated silicon sheet be sure to make some little holes in the bottom with a fork. This prevents the dough from raising up.

20 g	egg yolk
20 g	cream 35%

Meanwhile mix the egg yolk with the cream. We use this to make the tart shine and it helps retaining it’s crispy texture. Once the tart shell is done let it cool down for 5 minutes before removing the baking ring. If needed use a grader to thighty up the top and sides. This is optional, but it will give a better end result. The use a brush to add a thin layer of the glaze on both the inside and outside of the tart. Bake it for another 10 to 15 minutes till golden brown. After that let it cool down completely and keep it dry and covered for later.

The mustard sauce

4	shallots
40 g	butter
3 g	salt
2 cloves	garlic
250 g	white port
500 g	vegetable broth
500 g	cream 35%
2	bay leaves
50 g	coarse mustard

Begin by cutting the shallots in half and clean them. Then slice them in thin slices. After melt the butter on a medium heat and start glazing the shallots with the salt. Also clean the garlic and slice them in thin slices as well. Add the garlic to the pan and continue glazing till both the onion and garlic have softened. Then add the white port and reduce it till almost all the port has evaporated. It almost starts to become a bit sticky. When that’s the case add the vegetable broth with the cream, the bay leaves and the coarse mustard. Let this reduce on a soft simmer for about 30%. Once you’re happy with the flavor remove both the bay leaves and blend it till smooth. This will thicken the sauce and improve the flavor. Give it a taste for seasoning and keep it covered in your fridge for later.

The sweet and sour pearl onions

130 g	white wine
130 g	water
130 g	white wine vinegar
130 g	sugar
4 g	salt
3 sprigs	thyme
	Pearl onions

Mix the white wine with the water, the white wine vinegar, the sugar, the salt and the thyme. Bring this to a boil. Meanwhile cut some pearl onions and half and clean them. On some I also had to trim the top and bottom, but this is up to your preference. Once the sweet and sour is boiling add the cleaned onions and let it simmer for another minute. After the minute transfer both the onions and hot liquid into a preserving jar and close the lid. Let it cool down and then let the flavors infuse for at least 24 hours in your fridge. This will result in a fresh and sweet pearl onion with a lovely little bite.

Both the brioche toppings

The puffed mustard seeds

Start by seasoning boiling water with salt and boil some mustard seeds for around 30 minutes. If needed add some more water in between, just keep an eye on it. Once cooked transfer the seeds into a bowl of water and rinse the mustard seeds well to remove the slimy layer. If needed refresh the water once or twice. Then transfer the seeds onto a tray and spread them. Let them dry completely at 60 degrees Celsius. This is going to take a couple of hours. Every now and then you need to mix the seeds to prevent them from sticking to each other. When there almost dry you can rub them between your hands, this helps a lot as well. After that deep fry the seeds at 200 degrees Celsius for a couple of seconds. Place them in a sieve so you can easily take them out once puffed and crispy. When all the seeds float to the top you know they are ready to be taken out. Spread them on a tray that’s lined with a kitchen paper and season well with salt. Then let them cool down before you can keep it dry and covered for later.

The baked coarse mustard

100 g	neutral oil
200 g	coarse mustard

Begin with mixing the oil with the coarse mustard and while stirring bring this to a boil. Then keep on heating and stir it every now and then to prevent it from burning. Then moisture will slowly vaporize and the mustard will start to fry. Once the mustard start to color you need to turn down the heat for more control. This is the color we’re looking for. Now drain the mustard and press out any oil. Don’t throw this away, it’s packed with mustard flavor and perfect for a vinaigrette or to finish a hollandaise. Let it cool down and then keep it dry and covered for later.

The brioche

20 g	milk
4 g	dry yeast
250 g	plain flour
150 g	egg
30 g	sugar
10 g	onion powder or leek powder
3 g	salt
150 g	cold butter

First mix the milk with the dry yeast and heat it up till it’s at room temperature. Mix it well to dissolve the yeast. Then mix the plain flour with the egg, the sugar and the onion powder or leek powder. I use the leek powder I made for my last dish. Now start kneading it and then add the yeast mixture and the salt. Knead it on the lowest speed for 10 minutes. After that add the cold butter and knead it for another 5 minutes. Insure that all the butter is incorporated and that it’s a nice and elastic dough. Here you can also see the beautiful color that came from the leek powder. Now transfer the dough on your worktop and use a dough scraper to move the dough from one side of the worktop to the other. Really press it down so the dough will fold itself creating a beautiful ball. If the dough is sticking to much just add a splash of water on your hands. This will resolve that issue. Now transfer the dough in a bowl and cover it with some wrap or a damp cloth. Let it proof for 2 hours till doubled in size or let it rest overnight in your fridge.

Meanwhile line some baking rings with parchment paper. After that roll the dough into a long thin roll and portion it into pieces that weigh 12 grams. Then press the dough down with your hand and while circling, slowly raise your hand. With this movement you will create a nice small ball. Now place them in the baking rings. I place 5 balls in each ring. Cover them once more and let the dough proof for 45 minutes till doubled in size.

50 g	egg
50 g	cream 35%

Meanwhile mix the egg with the milk and set it aside. Then once the dough has proofed brush the top with the egg glaze before sprinkling both the toasted mustard and the puffed mustard seeds on top. Bake them at 190 degrees Celsius for around 15 to 18 minutes till golden brown. Once they’re golden take them out of the oven and directly glaze them with some melted butter. Then remove the rings and the parchment paper. Now try to serve them as fast as possible, but otherwise heat them up at 190 degrees Celsius for 2 minutes before serving.

The roasted onion & onion compote

5	cevenne onoins
10 g	honey
4 sprigs	thyme
80 g	vegetable broth
	salt & white pepper
80 g	butter
	Coarse mustard

First trim both the bottom and top of the cevenne onions and clean them. Now cut them in half. Both halves need to be around the same size, so keep an eye on that. Place them in a baking tray and drizzle the honey on top. Also add the leaves from the thyme, the vegetable broth and season it with white pepper and salt. Now cover it with parchment paper and then wrap it tightly with aluminum foil. Bake the onions at 180 degrees Celsius for 45 minutes. Let them cool down for 10 minutes before cutting them in half once more. Keep one half whole. Then line the bottom of a baking tin with parchment paper and start laying the onion layers from the outside to the inside. The tin is slightly bigger as tart we made before. Keep any onion trimmings for later. Finish the center with the whole half and then drizzle the cooking liquid on top. Also add the butter cubes and bake the onion for another 30 minutes at 180 degrees Celsius. This will cook the onions a little bit more and give a beautiful caramelization. Now let it cool down completely in your fridge.

Meanwhile take all the onion trimmings and chop it into a nice tartare. Don’t over chop it, you still want some texture. Transfer it into a pan and season it with coarse mustard, Just start with a half a tablespoon and add more to your liking. Then let it simmer on a medium heat for about 15 minutes. After that let it cool down completely. Now to finish the tart first cover the bottom with all the onion and mustard compote and level it. Then place a piece of parchment paper on the caramelized onions and use another tray turn it around. Remove the baking tin and press it into a right shape so it matches the size and shape of the tart. Now turn the tart around and gently place it on a caramelized onion. Then turn it around once more and remove the paper. Just look at all those beautiful layers! Now you could eat it all by yourself, but if you have to gently portion it to the desired size. I would use a very sharp slicer to insure you preserve those layers we worked so hard for. You can keep them covered in your fridge for a couple of hours.

How to finish the dish

To heat them up just reheat them in an oven at 190 degrees Celsius for 3 to 4 minutes. The sauce you can just bring to a low simmer. Now place the tart slice on a preheated plate and cover the top with a couple of the sweet and sour pearl onions. Also add some watercress, it will pair super well with the mustard. Then the dish is ready to be served and you only need to finish it with a hot mustard sauce.