Preheat oven to 425 F/220 C.
Separate cauliflower into individual florets of medium size and place in a large bowl. Drizzle the oil over the cauliflower and toss to coat.
Spread the cauliflower evenly on a baking sheet and sprinkle with salt and pepper. Roast for 15-20 minutes or until the cauliflower is golden. The time is approximate depending on how roasted you want it.
Place the cauliflower in a 2 qt/2 L baking dish and set aside.
Turn the oven temperature down to 400 F/200 C
To make the sauce: melt the butter in a medium saucepan and when it sizzles, whisk in the flour, cayenne (if using), nutmeg (if using), the mustard and salt. Whisk well so there are no lumps. Let this bubble for about a minute to cook the flour.
Slowly whisk in the hot milk and stir constantly until the mixture bubbles, then continue cooking for 1 minute. Remove from the heat then stir in all but 1/4 cup of the shredded cheese. Stir until cheese is completely melted.
Pour the cheese sauce over the cauliflower. Sprinkle with breadcrumbs and the remaining cheese.
Bake for 15-20 minutes or until topping is crisp and golden.