HEAT oven to 350°F.
COOK macaroni as directed on package, omitting salt. Meanwhile, cook onions in butter in large saucepan on medium heat 3 min. or until crisp-tender, stirring occasionally. Add flour; cook and stir 1 min. Add milk; cook 5 min., stirring constantly. Simmer on low heat 10 min., stirring frequently. Remove from heat. Stir in 2 cups cheese.
DRAIN macaroni. Add to cheese sauce; toss to coat. Spoon into casserole dish sprayed with cooking spray; sprinkle with remaining cheese.
BAKE 20 min. or until heated through.
serving size = 1 cup (250 mL)