Separate the cauliflower into small florets and put in a steaming basket over about 1/2 inch of simmering water. Cover and let the cauliflower steam for about 5 to 8 minutes, until crisp-tender or tender, depending on how you like it.
Meanwhile, prepare the cheese sauce.
Melt the butter in a saucepan over medium heat. Add the flour, ground mustard, and nutmeg and cook, whisking or stirring constantly, for about 1 1/2 to 2 minutes.
Gradually whisk in 3/4 cup of the milk or half-and-half and continue cooking, stirring constantly, until thickened.
Stir in the shredded cheese and continue cooking, stirring, until the cheese has melted.
Thin the sauce with a tablespoon or two of milk, if desired.
Add salt and freshly ground black pepper, to taste.
Makes 1 1/4 cups of sauce.
Serve the cauliflower and pass the sauce separately.