Dust the assembled dish with paprika if you want a little sweetness and color, or Aleppo pepper if you prefer some heat. Slideshow: More Cauliflower Recipes
Preheat the oven to 350°. Butter a large gratin dish.
Heat the 3 tablespoons of butter in a large saucepan over moderate heat. Add the flour and mustard and cook, stirring until smooth, about 1 minute. Add the milk and cook over moderately low heat, stirring occasionally, until thickened, about 10 minutes. Add the cheese and stir until melted and completely smooth. Season the sauce with salt and pepper, and keep it warm over very low heat.
Meanwhile, using a strainer insert in a large pot of boiling well-salted water, cook the cauliflower until crisp-tender, 3 to 5 minutes. Strain and set aside. Add the pasta to the boiling water and cook until al dente. Drain well.
Add the cauliflower and pasta to the cheese sauce and stir to coat completely. Pour the mixture into the buttered gratin dish and top with the bread crumbs. Sprinkle some paprika over the dish.
Bake until bubbling and golden, 20 to 25 minutes.