PREHEAT oven to 350°F. Cook potatoes in large pot of boiling water for 5 minutes; drain. Meanwhile, brown meat with onion in large nonstick skillet, stirring occasionally. Drain, if necessary. Add spaghetti sauce to skillet.
MIX ricotta cheese, grated topping and egg whites until well blended. Spoon half of the meat mixture into 8-inch square baking dish sprayed with cooking spray. Cover with half of the potatoes; top with layers of the ricotta cheese mixture, remaining potatoes and remaining meat mixture. Sprinkle with mozzarella cheese. Cover with foil.
BAKE 50 minutes. Remove foil. Bake an additional 10 minutes or until cheese is melted.