In a skillet over medium heat, melt butter. Add apple rings, sprinkle with 2 teaspoons sugar cook until tender, turning once. In a mixing bowl, beat the egg yolk, lemon peel and vanilla for 1 minutes.
In another mixing bowl, beat egg white until stiff peaks form, fold in flour and remaining sugar. Fold into egg yolk mixture. Place apple rings in a greased 2 cup baking dish. Spread egg mixture on top. Bake at 350 degrees for 15-18 minutes or until golden brown and set. Invert onto a serving plate.
