Steam the cauliflower for about 10-15 minutes. I like it a bit crispy. Drain and put in a 9"x9" pan sprayed with Pam.
In small bowl add mayonnaise, small amount of wet mustard. Whisk until smooth. Add a little milk and whisk until you can pour it over the cauliflower. Grate cheese - I use old cheddar. Cover the cauliflower dish with lots of cheese.
Put the baking dish in the oven and broil until the cheese is melted and browned.