Preheat the oven to 350 degrees F (175 degrees C).
In a small pot, heat the soup, butter or margarine and sour cream over low heat.
Combine potatoes, green onion and 1 cup cheese into a large mixing bowl. Mix in the heated soup mixture. Season with salt and pepper. Pour into a 9x13 inch dish. Sprinkle one cup of cheese over the top of the casserole.