Melt 2 Tbsp. margarine in large saucepan on medium heat. Add onions; cook and stir 3 min. or until crisp-tender. Stir in flour and mustard; cook and stir 1 min. Whisk in milk; cook 5 min. or until thickened, stirring frequently. Remove from heat. Add 1-1/4 cups cheddar and 3 Tbsp. Parmesan; stir until cheddar is melted. Add cauliflower; stir to evenly coat. Spoon into 2.5-L casserole dish sprayed with cooking spray.