Combine sugar, flour, salt and margarine. Add lemon juice and zest. Stir in egg yolks and milk. Mix well. Fold in egg whites. Pour into a greased (one quart) casserole and bake in a larger pan of water at 160ºC for 1 hour. Cool (and chill) before serving. When done there is a meringue-like topping with a sponge cake center and a pudding of cream-like consistency on the bottom, so it can't be unmolded.