Blanch the cauliflower in salted water 5 minutes. Rinse and drain.
Make the sauce: melt the butter in a saucepan. Add the flour and stir 1 minute but don't let it brown. Remove from heat and slowly whisk in the milk.
Return the milk to the heat and simmer very gently about 10 minutes. Keep stirring so it doesn't stick.
Remove from heat again and stir in about 3/4 of the cheese, the cream and the mustard. Season with nutmeg, salt and pepper. Careful not to make it too salty!
Place the cauliflower in a baking dish and pour over the sauce. Sprinkle with the remaining cheese and breadcrumbs. Bake 30 minutes or until brown and bubbling.