Place cauliflower in 1-1/2-quart casserole with cover.
Add 2 tablespoons water.
Cook, covered, on HIGH 7-8 minutes, or until just about tender.
Combine mayonnaise, onion and mustard, mixing well.
Place cooked cauliflower in serving dish.
Spread mayonnaise mixture over cauliflower.
Sprinkle with cheese.
Cook, uncovered, on HIGH for 1 minute to heat topping and melt cheese.