Nonfat cooking spray
1 medium sweet onion, peeled and sliced into 1-inch strips
8 Granny Smith apples, peeled and sliced into 1/8-inch slices
1/3 cup reduced-calorie stick margarine
3/4 cup brown sugar, packed
4 egg whites
3/4 cup plus 2 tablespoons flour
1-1/2 teaspoons baking powder
5 ounces skim milk
3/4 cup plus 2 tablespoons sugar
Preheat oven to 350 degrees. Liberally spray two baking sheets with nonfat cooking spray. Place onions on one baking sheet and apple slices on the other. Bake onions until caramelized, for about an hour. Bake apples for about 4-5 minutes; remove from oven and set aside.
Melt margarine in a 10-inch skillet and add brown sugar. Stir with wooden spoon until dissolved. Remove pan from heat, add caramelized onions and set aside.
In a large bowl, beat egg whites until foamy and add flour, baking powder, milk and sugar until well mixed. Spray a 9-inch baking pan with nonfat cooking spray and then line with parchment paper. Layer brown sugar-onion mixture on parchment paper. Place the sliced apples over the onions. Press down to fill gaps between the apples.
Cover with batter and bake about 45 minutes or until a toothpick inserted in the center comes out clean. Remove from the oven and cool to room temperature. Reverse the pan onto a serving tray and lightly dust with confectioner’s sugar.