Jan's Au Gratin Potatoes

1-1 1/2# Red Potatoes, thinly sliced
Onion, finely chopped
8T Butter, cut into cubes
2c Shredded Sharp Cheddar Cheese
1c Shredded Parmesan Cheese
Salt & Pepper to taste
1/2c Milk

Butter a small casserole dish. Layer thinly sliced potatoes, chopped onions, butter cubes, salt, pepper, Cheddar cheese and Parmesan cheese. Repeat for a total 3-4 layers. Pour the milk over everything. Bake at 350° for at least an hour or until potatoes are tender. Serves 4