Mix the frozen strawberries with the sugar. Now cover it well and cook it on a double boiler for 3 to 4 hours. After that remove the wrap and let the liquid drain on a sieve that’s lined with a kitchen paper. Do this in your fridge.
Then transfer the strawberry broth into a saucepan together with the champagne and the agar powder. Then mix it and bring it to a boil for at least 1 minute. After, pour it into a bowl and let it cool down in your fridge. Once it’s cold and it has set, transfer it into a blender and blend it till smooth.