Preheat oven to 375 degrees. Butter a shallow, 1.5 to 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, 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 handful for later. Stir until cheese is completely melted. Stir in the dry mustard, nutmeg, salt and pepper.
Spread the cooked cauliflower evenly in the casserole dish, and pour the cheese sauce over the top.
Sprinkle the remaining cheese over the top.* Bake until bubbly and lightly browned, about 10 minutes. Allow to cool slightly before serving.
The sauce thickens as it cools.
*For added texture, you could mix 3 tablespoons of bread crumbs with the remaining grated cheese before adding to the casserole.