Preheat the oven to 400 degrees F. Blanch the cauliflower in a large pot of boiling salted water for 5 minutes, until still crisp, then drain into an ice bath.
Melt the butter in a saucepot over medium heat. Once it foams and subsides, add the flour and cook, whisking, 3 minutes. Add the hot milk and whisk constantly as it comes to a boil and thickens.
Off the heat, add a pinch of salt, a few cracks of pepper, the nutmeg and the cheese and stir until melted.
Pour 1/3 of the sauce into a lightly greased 9 by 9 casserole, add a layer of cauliflower and more sauce, then another layer and the rest of the sauce. Top with more cheese.