This is my absolute favorite dish that my mom makes. The onions have a mild sweet flavor, and there's plenty of the rich creamy sauce. The sweetness makes a nice complement to the Peppered Beef Tenderloin.
In a large skillet or Dutch oven, saute onions in butter until tender and golden brown, about 25 minutes. 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 minutes or until thickened. Reduce heat to medium. Return onions to the pan; heat through.