Preheat oven to 375. Prepare a 8" x 8" pan with cooking spray; set aside. Sprinkle 3/4 cup ham in bottom of prepared pan; set aside. In a skillet, cook onions until tender. Remove and sprinkle over ham in pan. Sprinkle cheese over ham and onions. In a mixing bowl, combine yogurt and cornstarch. Mix until creamy. Add egg whites, salt, and black pepper. Mix well. Pour over ham mixture in pan. Sprinkle with remaining ham. Bake for 35 minutes, or until knife inserted in center comes out clean. Let stand ten minutes before serving.