HEAT oven to 350°F.
BRING large saucepan of water to boil. Add spinach; cook 1 to 2 min. or just until wilted. Drain. Use same pan to cook macaroni as directed on package, omitting salt. Meanwhile, melt margarine in large saucepan on medium heat. Add onions; cook 5 min., stirring frequently. Add garlic and flour; cook and stir 1 min. Whisk in milk; bring to boil, stirring constantly. Add 2-1/4 cups cheese; cook and stir until melted. Stir in pepper and nutmeg.
SQUEEZE excess water from spinach; chop finely. Drain macaroni. Add to cheese sauce; stir until evenly coated. Spoon half the macaroni mixture into 13x9-inch baking dish; cover with spinach. Top with remaining macaroni mixture and shredded cheese.
BAKE 30 min. or until golden brown. Let stand 10 min. before serving.