PREHEAT oven to 350°F. Cook macaroni as directed on package. Meanwhile, cook onions in butter in large saucepan on medium heat 3 min. or until tender, stirring occasionally. Add flour; cook and stir 1 min. Add milk; cook 5 min., stirring constantly. Reduce heat to low; simmer 10 min. Remove from heat. Stir in 2 cups of the cheese.
DRAIN macaroni. Add to cheese sauce; toss to coat. Spoon into greased casserole dish; sprinkle with remaining 1 cup cheese.
BAKE 20 min. or until heated through.