Peel potatoes and chop into quarters. Set them in a large pot and cover with water, allowing it to come to a boil. Boil potatoes until tender, about 30 minutes. While potatoes are boiling, cook pasta according to directions, and once drained mix with two tablespoons of butter. Set aside. In a large skillet over medium heat, melt two tablespoons of butter and saute chopped onion until soft.