Cut cone shape wedge out of cauliflower core. Place in 1 1/2 qt. microwaveable casserole dish with 1/4 cup water. Cover and cook on HI (max power) for 9 minutes. Drain moisture.
Mix mayonnaise, onion, mustard and salt. Spoon sauce over cauliflower. Lay cheese slices over top of cauliflower. Cook, uncovered, on 70% (roast) for 1 minutes or until cheese is melted. Sprinkle top with paprika.