Preheat oven to 375°F (190°C). Lightly butter the bottom and sides of a shallow 2-quart casserole dish.
Place the cauliflower pieces in a steamer basket. Place the basket over simmering water in a large pot, cover and cook until fork tender but not soft, about 10 to 15 minutes.
Melt the butter in a medium saucepan over medium-low heat. Whisk in the flour and cook for 2 to 3 minutes to create a roux.
Slowly pour in the milk, constantly whisking to combine until completely smooth and all the milk has been added. Bring mixture to a simmer, whisking occasionally until sauce thickens and coats the back of the spoon, about 2 to 3 minutes.
Add the cheese a bit at a time, reserving a quarter cup for later. Stir until cheese is completely melted. Stir in the dry mustard, nutmeg, salt and pepper.
Pour a little less than half of the cheese sauce into prepared baking dish, and spread the drained cauliflower evenly over the sauce. Pour the remaining cheese sauce over the top. Sprinkle the remaining cheese over the top.
Place cauliflower bake on a baking sheet and bake until bubbly and lightly golden brown, about 20-25 minutes. Allow to cool slightly before serving and sprinkle with fresh parsley. Can be served hot or at room temperature.
Notes
The sauce thickens as it cools.
For added texture, you could mix 4 tablespoons of bread crumbs with 2 tablespoons of melted butter with the remaining grated cheese before adding to the casserole.
You can use frozen cauliflower, just make sure to thaw and drain them well before using.
Aged, sharp cheeses add the best flavor. White or yellow cheddar cheese, Gruyere cheese, Parmesan cheese, or a combination of these are all delicious options.
See original post for made in advance and storage options.