For 4 people
Wash the cythera apples in clean water and then peel them to remove their skin. Remove all the flesh covering the thorny core.
Place the resulting pulp in a blender or juicer. Add lemon juice if desired. Blend until smooth.
Pour this purée into a fine sieve and use the back of a spoon to extract as much liquid as possible and separate the purée from the fibrous minced residue. This will give you a Cythera apple coulis.
Make the sugar syrup by putting the sugar and water in a saucepan.
Bring everything to a boil until simmering.
Pour this syrup into the resulting coulis. Mix everything well.
Depending on the desired juice density, add a little water to the mixture. Stir one last time.
Pour into a carafe or pitcher and refrigerate.