Thaw potatoes by leaving them out for 30-45 minutes. Preheat oven to 350.

In a large mixing bowl, mix together the potatoes, sour cream, cream of onion soup and cheese.

Melt butter in microwave and pour over the mixture and mix thoroughly.

Transfer mixture to a 9x13 baking dish and heat in oven for one hour, then broil for 5-10 minutes to brown the top.