Homemade Macaroni & CHeese

1 1/2 cup elbow noodles
3 tabl butter
1/4 chopped onion (optional)
2 tabl Flour
1/2 tea. salt
pepper
2 c Milk
2 c Sharp american cheese
more salt

Cool noodles till done.In saucepan add butter & flour & salt & peper.Add milk cook until thickened ( I wisk all of this)then add butter it calls for cubed cheese but shredded works just as well.stir until cheese is smooth texture, then dump noodles in cassarole dish and pour cheese mixture ontop & stir, if I have left over cheese i sprinkle ontop of noodles.Bake uncovered 30 min at 350!