Stir the potatoes, soup, sour cream, black pepper and onion in a medium bowl. Spoon the potato mixture into a 1 1/2-quart baking dish.
Bake at 350°F. for 30 minutes or until the potato mixture is hot.
Tip: To make 3 cups mashed potatoes, place 2 pounds potatoes, peeled and cut into 1-inch pieces, into a 3-quart saucepan. Add water to cover and heat over medium-high heat to a boil. Reduce the heat to low. Cover and cook for 10 minutes or until the potatoes are tender. Drain. Mash the potatoes with 3/4 cup milk and 2 tablespoons butter.