1Heat oven to 400°F. Spray 1 1/2-quart casserole or square baking dish, 8x8x2 inches, with cooking spray.
2Mix mashed potatoes, onions, milk, salt, pepper, nutmeg and 1/4 cup of the cheese in large bowl. Lightly spoon into casserole. Sprinkle with remaining 1/4 cup cheese.
3Bake uncovered 15 to 20 minutes or until potatoes are thoroughly heated and cheese is melted.