Heat oil in medium saucepan on low heat 1 min. Add onions; cook 2 to 3 min. or until softened, stirring occasionally. Add flour; cook and stir 2 min. Gradually add milk, stirring until well blended after each addition. Bring to boil on medium heat; cook 2 to 3 min. or until thickened, stirring constantly. Add to macaroni mixture; toss to coat.