Cut the cauliflower into small flowerets and steam until just fork tender (about five minutes in the microwave).
Prepare a bechamel sauce: Melt the butter in a small saucepan and stir in the flour. Cook on low heat stirring for several minutes, or until the roux just starts to brown. Whisk in the milk gradually to form a smooth sauce. Warm until just simmering.
Remove the sauce from the heat and gradually stir in about half of the cheese, the pepper and the nutmeg.
Place the steamed cauliflower in a buttered baking dish and cover evenly with the cheese sauce. Sprinkle with remaining cheese and bake for 25 minutes at 350°F or until golden brown.