Have cheese, sour cream, butter, and onion at room temperature.
Bake potatoes at 425° till done, 45 to 60 minutes.
Meanwhile, in small bowl combine cheese, sour cream, butter, and onion; beat till fluffy.
To serve, roll potatoes gently under hand.
Immediately criss-cross tops with fork; press ends, pushing up to fluff.
Top with cheese mixture.