In a saucepan, melt butter. Cook onion in butter until translucent.
Stir in flour, salt, pepper, and milk.
Stirring frequently, cook until sauce thickens.
Stir in cheese and allow to melt.
Cut potatoes into rounds. Place half of potatoes in greased 9x13.
Pour half of sauce over potatoes.
Repeat with other half of potatoes and sauce.
Top with more cheese if desired.
Bake covered at 350° for 45 minutes.
Uncover potatoes and stir.
Bake uncovered for 30 minutes or until potatoes are soft.
Notes
You can change the flavor of this by changing which cheese you use. I like a combination of cheddar and mozzarella, and I always put a little more on top for a nice layer of cheesy goodness!