Cook macaroni as directed on package, omitting salt. Meanwhile, heat oil in large saucepan on medium-high heat. Add onions and garlic; cook and stir 5 min. or until crisp-tender. Stir in flour and mustard; cook and stir 1 min. Gradually whisk in milk, cooking and stirring after each addition until slightly thickened. Simmer on low heat 5 to 7 min. or until thickened, stirring constantly. Remove from heat. Add 2-2/3 cups cheese; stir until melted.