Place cauliflower florets in a large glass bowl or casserole dish and cover with plastic wrap. Poke a few holes in the plastic with a knife. Cook in the microwave for 5 to 7 minutes or until tender.
In a cup or small bowl, stir together the mayonnaise, and Dijon mustard. Spread over the top of the cauliflower. Sprinkle grated cheese over the top. Return to the microwave, and cook just until cheese has melted, about 2 minutes.