Au gratin Potatoes

Au gratin Potatoes
2 lbs. potatoes, sliced
1 medium onion, chopped
1/4 c. butter
1 T. flour
1 t. salt
2 c. milk
2 c. (8oz.) shredded sharp cheese
Boil potatoes for 25-30 minutes. Meanwhile, in a saucepan, cook & stir onion in butter over low heat until onion is tender. Add flour & stir. Add milk and 1 1/2 c. cheese. Boil & stir 1 minute.
Place potatoes in 1 1/2 quart casserole dish. Pour sauce on potatoes. Cook at 375 for 1 hour or at 325 for 1 hour 20 minutes. Add the remaining cheese during the last 20 minutes.