Preheat oven to 350 degrees. Liberally spray 2 baking sheets with nonfat cooking spray. Place onions on one baking sheet and apple slices on the other. Bake onions until caramelized, for about 1 hour. Bake apples for about 4-5 minutes; remove from oven and set aside. Heat oil in a 10-inch skillet add brown sugar and cinnamon. Stir until dissolved. Remove pan from heat, add caramelized onions and set aside. In the large bowl of an electric mixer, beat egg whites until foamy and add flour, baking powder, milk and sugar until well incorporated. Spray a 9x13-inch baking pan with nonfat cooking spray. Layer brown sugar-onion mixture in prepared pan. 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 center tests clean. Remove from oven and cool to room temperature. Reverse the pan onto a serving tray and lightly dust with confectioner's sugar.