Trim the cauliflower and break into small florets. Boil in salted water for 10-15 minutes or until tender. Drain the cauliflower and place in buttered ovenproof baking dish.
Mix together milk, flour and butter in a saucepan. Heat and whisk continuously until sauce thickens and is smooth. Allow sauce to simmer for another 2 minutes.
Add ¾ of the cheese, mustard, pinch of nutmeg, and seasoning. Cook one minute, stirring well. Pour sauce over the cauliflower. Mix remaining cheese and the breadcrumbs together and sprinkle over the top.
Bake for 15 to 20 minutes, until brown and bubbling. Serve immediately.