Cook the apple slices in about 2 tablespoons of water, stirring occasionally until they form a puree. Add about 3/4 of the sugar, the lemon juice, and the egg yolks: mix well. Put into an ovenproof dish and bake at 350°F for 20 minutes. Meanwhile, whisk the egg whites until stiff, folding in the remaining sugar. Pile the meringue evenly on top of the apple mixture, return the dish to the oven, and bake for about 10 minutes. Serve hot or cold.