Preheat oven to 350 degrees.
Peel, core, and slice apples. Cook in saucepan to make a purée. Add sugar.
Cook till dry and thick. Cool mixture.
Beat egg whites and cream of tartar in an electric mixer.
When firm, add sugar and continue beating for 1 more minute at medium speed.
Fold in apple mixture. Add Grand Marnier.
Make a caramel with sugar, a little water and lemon juice.
Pour in a soufflé mold. Pour apple mix in mold.
Bake in waterbath for 30 minutes. Unmold mousse on a serving plate.
Serve with custard cream.
Mix sugar and egg yolks till creamy. Gradually whisk in boiling milk.
Add vanilla. Heat in a saucepan and stir with a wooden spoon over low heat till thick.
Do not allow to boil. Strain and cool.