Clean cauliflower & pierce the stem with a knife so it cooks more quickly. Cook for about 5-8 min. on high in a covered microwave safe dish. While it is cooking, combine mayonnaise, mustard, a little salt & pepper. Spread this over the cauliflower after it is finished cooking, then sprinkle grated cheddar cheese on top. Put the cover back on & let it stand for about 5 minutes; serve.