1Heat oven to 350°F if using julienne potatoes or 400° if using au gratin potatoes. Spray 13x9-inch (3-quart) glass baking dish with cooking spray.
2In large bowl, mix Potatoes, Sauce Mix (from both potato mixes) and boiling water. Stir in sour cream, milk, margarine, bell pepper and 1/4 cup of the onions. Pour into baking dish.
3Bake uncovered 35 minutes. Sprinkle with remaining onions and the cheese. Bake 5 to 10 minutes longer or until bubbly around edges and potatoes are tender. Let stand 5 minutes before serving.