In a large bowl, thaw potatoes and fluff with a fork.
Add margarine, salt, pepper, onion, undiluted soup,
sour cream and cheese; mix well.
Into a greased 9x13" baking dish, place mixture.
Top with shredded cheese and bake at 325F
until potatoes are done, about 1 hour.

As an alternative, mix in 1 c. shredded cheese
with other ingredients and top with remaining
cup of cheese before baking. Or, mix all the cheese
in with the other ingredients and top casserole with
finely crushed corn flakes just before baking.
Makes 10-12 servings.