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 3 cups cheese; stir until melted.
Heat oven to 350 degrees F.
Drain macaroni.
Add to cheese sauce; toss to coat. Spoon into 2.5-L casserole dish; top with remaining cheese.