Bring a large pot of salted water to a boil and add potatoes. Cook until tender, then drain water. While potatoes are cooking, heat a small skillet over medium heat. Melt butter and saute onion until golden brown. Set aside.
Preheat oven to 350 degrees F (175 degrees C).
Mash potatoes with cream of chicken soup and sour cream until smooth and creamy. Mix in onions, 3/4 cup cheese, salt, and pepper. Spoon into a 2 quart casserole dish and bake in preheated oven for 30 minutes. Sprinkle remaining 1/4 cup cheese over top and bake an additional 10 minutes. Serve hot.