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 minute. 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 180ºC or 15-18 minutes or