Spread noodles in buttered 13×9 casserole. Stir tuna and broccoli together; layer over noodles. Top with half the cheese. Blend soup and water; pour over casserole. Sprinkle with remaining cheese. Bake at 350° 25 minutes (or 8 minutes on Med.- High in microwave). Top with onions and bake 5 minutes more (microwave 2 minutes); let stand 5 minutes before serving.