Cut cauliflower into 8-12 large pieces, discarding the core. Place in microwaveable casserole dish with larger pieces on the outside of the dish. Add water;cover. Microwave on high 10 minutes; drain. Return cauliflower to casserole dish.
Mix mayo and mustard. Spread over cauliflower; sprinkle with cheese. Microwave on high 1-2 minutes or until cheese melted.