Preheat oven to 350.

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.

Drain potatoes and mash with one stick of butter and cheese. Make sure no lumps remain. Fold in onions.

Spread out pasta in a large baking dish and spread mashed potato mixture over top. If desired, top with more cheese and onions. Bake at 350 for about 25 minutes. This is best when served immediately, but still tastes good after being in the fridge for a day or two.