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