Place 1 head cauliflower florets in a 2-quart baking dish. Whisk 1/2 cup heavy cream, 1/2 teaspoon dry mustard and 1 teaspoon salt, then pour over the cauliflower. Sprinkle 1 cup shredded gruyere and 1/3 cup breadcrumbs on top. Bake at 350 degrees F until tender, about 45 minutes.
Photograph by Antonis Achilleos