Cook cauliflower in boiling water in large saucepan 4 minutes; drain well.
Melt 2 Tbsp. margarine in large saucepan on medium heat. Add onions; cook and stir 3 minutes or until crisp-tender. Stir in flour and mustard; cook and stir 1 minute. Whisk in milk; cook 5 minutes 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.
Microwave remaining margarine in medium microwaveable bowl on HIGH 30 sec. or until melted. Stir in bread crumbs and remaining Parmesan; sprinkle over cauliflower mixture.
Bake 20 to 25 minutes or until heated through, topping with remaining Cheddar for the last 5 minutes.
