Peel potatoes and cut into thin slices.
Make the white sauce by melting the butter in a large skillet. Over medium heat, add the flour and brown. Add the milk and continue to cook until the sauce thickens.
Grease a 2 quart casserole dish and add 1/2 of the potatoes.
Cover potatoes with 1/2 of the sauce, seasonings and onion. Add remaining potatoes, seasonings and onion, the cover with the remaining sauce.
Cover and bake at 350 °F for 1 hour. Uncover, and continue to bake until the top is browned.