Place cauliflower in 13x9-inch baking dish sprayed with cooking spray. Melt margarine in medium saucepan on medium heat. Add onions; cook and stir 3 min. or until crisp-tender. Stir in next 3 ingredients. Gradually whisk in milk. Bring to boil; cook and stir 2 min. or until thickened. Add 1 cup cheese; cook 2 to 3 min. or until melted, stirring constantly. Pour over cauliflower.