Spray a 1-quart casserole dish with cooking spray.
Trim the cauliflower and cut into florets.
Steam the cauliflower in a large pot with a little water over medium-low heat until tender. Remove the cauliflower from the pot to a towel to remove excise moisture.(Or you could roast the cauliflower for about 30 minutes, turning once, at 400*F.).
Heat the butter in a medium saucepan over medium heat. When the butter has melted, whisk in the flour. Cook, stirring, for 2 minutes.
Add the milk, whisking all the time to make sure there are no lumps.
Cook another 3 to 4 minutes, and let the sauce thicken. Season with salt and pepper, to taste and add the nutmeg.
Put the cauliflower into the prepared pan. Top with half of the cheese. Pour the white sauce (bechamel) over the cheese. Top with the remaining cheese and the bread crumbs.
Bake, uncovered for 20 to 30 minutes until golden and bubbly.