Steam whole head of cauliflower for 30 minutes. Drain and place on a pie plate.
Preheat the oven to 375 degrees F (190 degrees C).
In a mixing bowl, combine mayonnaise and mustard; spread the mixture over cauliflower and dot with pats of butter. Sprinkle generously with Parmesan cheese.
Bake in the preheated oven uncovered, for about 30 minutes, or until cheese is brown.