Heat oven to 350°. Spray rectangular baking dish, 11x7x1 1/2 inches, with cooking spray.
Make potatoes as directed on package for 8 servings, using 2 pouches Potatoes and Seasoning, 2 cups water, the milk and butter. Spread half of the potatoes over bottom of baking dish. Layer with beef, 1 cup of the cheese and remaining potatoes.
Cover with aluminum foil and bake about 30 minutes or until hot. Sprinkle with remaining 1/2 cup cheese. Sprinkle onions around edges of baking dish. Bake uncovered 3 to 4 minutes or until cheese is melted and onions are brown.