Place potatoes and onion in water until covered and boil until potatoes are tender. Drain and set aside. Melt butter in Large pot over medium heat. Whisk in flour until smooth, then gradually stir in milk. Bring to a boil, and simmer for about 5 minutes until slightly thickened. Add potatoes and onions that you set aside. Stir in sour cream and cheddar cheese. Season with salt and pepper. Once your cheese and sour cream has melted remove from heat and serve.