Preheat the oven to 350 degrees F. Grease a 9 x 13-inch baking pan. Spread the potatoes in the pan. Season with salt and pepper. Melt the butter in a medium bowl in the microwave. Add the onions, sour cream, and soup and mix together. Pour the soup mixture over potatoes. Sprinkle with the cheese. Bake for 45 minutes, or until all is completely warm and the cheese begins to melt.