Heat oven to 375°F. Cook and drain macaroni as directed on package using minimum cook time.
Meanwhile, in 2-quart saucepan, melt butter over medium heat. Cook onion, salt and pepper in butter, stirring occasionally, until onion is crisp-tender.
In small bowl, mix flour and milk until smooth; stir into onion mixture. Heat to boiling, stirring constantly. Boil and stir 1 minute; remove from heat. Stir in cheese until melted. Stir in macaroni. Into ungreased 1 1/2-quart casserole, spoon macaroni mixture.
Bake about 30 minutes or until bubbly and light brown.