Cheddar Potato Bake

1 can Campbell's condensed Cheddar cheese soup
1/3 cup sour cream or plain yogurt
2 tablespoons chopped scallion
Generous dash pepper
3 cups STIFF, seasoned mashed potatoes

In 1 1/2-quart casserole, combine soup, sour cream, onion and pepper. Stir in potatoes until blended. Bake at 350 degrees F for 30 minutes or until hot and bubbling. Garnish with shredded Cheddar cheese and sliced scallion, if desired.