Cut the potatoes into thin slices.  Mix the potatoes, chopped onions, season salt and pepper in a bowl.  Place half the potatoes in a casserole dish.

 Meanwhile melt butter in a saucepan.  Blend in flour.  Add milk all at once; cook stirring constantly, until slightly thickened.  Add shredded cheese.  Season with a small amount of salt and pepper.  

Spread half the cheese mixture over the potatoes in the casserole dish.  Add the 2^nd half of the potatoes and cover with the remaining cheese sauce.

Bake at 350 degrees for 1 hour or until potatoes are tender.