HEAT oven to 350°F.
COOK macaroni in large saucepan as directed on package, adding spinach for the last minute. Meanwhile, cook bacon in large nonstick skillet until crisp, stirring occasionally. Remove bacon from skillet; drain on paper towels. Discard drippings from skillet.
ADD onions to skillet; cook and stir with whisk 3 to 5 min. or until crisp-tender. Stir in flour; cook and stir 1 min. Gradually whisk in milk. Bring just to boil, stirring constantly; cook and stir 3 to 5 min. or until thickened. Stir in 1 cup cheese; cook and stir until melted. Add bacon; mix well.
DRAIN macaroni mixture. Add to sauce; mix well. Spoon into 2-L casserole; top with remaining cheese.
BAKE 20 min. or until heated through.