Meanwhile, melt butter in medium saucepan on medium heat. Add onions; cook and stir 4 min. or until tender. Stir in flour; cook and stir 1 min. Gradually whisk in milk. Bring to boil, stirring constantly. Simmer on medium-low heat 3 to 5 min. or until thickened, stirring constantly. Add cheese; cook and stir 2 to 3 min. or until melted.