Put a large pot of water on to boil and salt it heavily. (Taste it: if it's not as salty as seawater, add more salt.)
While the water is coming to a boil, melt the butter in a medium pan over medium heat and sauté the garlic until it's fragrant, about 2 1/2 minutes. Add the bread crumbs and stir them in the butter and garlic just until they're coated, 30 to 45 seconds. Set the bread crumbs aside.
Cut off the end of the cauliflower. Discard any raggedy leaves; the other leaves are fine to use. Cut the head into very small florets and, if you like, thinly slice the stalk.
Check the pasta packaging for the cooking time. Stir the pasta into the pot of boiling water. When there are 8 minutes left, add the cauliflower. If the pasta takes less than 8 minutes to cook, start with the cauliflower, then add the pasta at the right time (the cauliflower needs to cook 8 minutes).
Heat the oven to 400°. Meanwhile, on top of the boiling pot, place a heat-proof bowl with the Cheddar and sour cream in it. Stir the mixture occasionally, as the heat from the pot melts the cheese. If the pot threatens to boil over, turn down the heat a bit. Season the sauce with pepper to taste.
Remove the sauce carefully (use oven mitts) and set it aside. When the pasta is tender, reserve 1/2 cup of the cooking water, then drain the pasta and cauliflower. Return them to the pot, add the cheese sauce and the reserved pasta water, and stir to combine. Add salt to taste.
Put the pasta and cauliflower in a buttered 9- by 13-inch baking pan, sprinkle the bread crumbs over the top, and pop the pan into the oven. Bake until the bread crumbs are brown and the sauce is bubbling, 12 to 15 minutes. Serve the dish hot.