In lg skillet or Dutch oven, saute onions in butter until tender and golden brown, about 25 minute Remove with a slotted spoon. Add flour, salt and pepper to skillet; stir until smooth. Gradually stir in milk until blended.
Bring to a boil; cook and stir for 2 min or until thickened. Reduce heat to med. Return onions to the pan; heat through.