Melt butter in skillet, add mushrooms and cook about 12 mintues. Heat 3 tablespoons of oil in a pan and fry potatoes until golden about 20 minutes. Season with salt and pepper. Add remaining oil to a skillet and saute onion for 15 minutes until golden brown. Stir mushrooms, onions and sour cream into the potatoes. Toss well. Cook over low heat until sour cream is heated through about 2 minutes. Serve.