Remove the leaves of the cauliflower and trim the bottom so it will sit flat. Place in a large microwave-safe dish, preferably one with a cover. Add water and cover (use plastic wrap with a couple of tiny holes to allow steam to vent if your dish does not have a cover).

Microwave on high 8 – 9 minutes or till tender crisp. Use a paring knife to test doneness. If you encounter resistance, return to microwave for 1 – 2 minutes longer. Remove the cauliflower from the microwave and uncover. Carefully drain any remaining water from the dish.

BE VERY CAREFUL, there are multiple ways you could burn yourself. The dish will be very hot. The steam escaping from the cover/plastic wrap can burn you. There will be boiling water in the bottom of the dish.

While the cauliflower cooks, combine mayonnaise, onion, and mustard in small mixing bowl. Set aside.

Spread the mustard sauce evenly over the top and sides of the steamed cauliflower.

Sprinkle the cheese over the cauliflower, gently pressing it into the mustard sauce so it will stick.

Microwave for an additional 1 – 2 minutes, uncovered, until the cheese melts.

Let stand two minutes before serving.