Meanwhile, melt butter in large saucepan on low heat. Add onions; cook and stir 3 to 5 min. or under crisp-tender. Whisk in flour and mustard; cook 2 min. Gradually whisk in milk until blended; cook on medium heat 3 to 5 min. or until thickened, whisking constantly. Add 2 cups cheese; cook 1 min. or until melted, stirring constantly.