Preheat oven to 475 degrees, with racks in upper and lower thirds. Set a large pot of salted water to boil. On a rimmed baking sheet, toss cauliflower, onion, and garlic with 2 tablespoons oil; season with salt and pepper. Roast on lower rack until lightly browned and tender, 20 minutes, tossing once.
Meanwhile, in a food processor, combine bread and remaining 2 tablespoons oil; pulse until coarse crumbs form. Spread on a rimmed baking sheet. Bake on upper rack until golden brown, 5 to 6 minutes, tossing once.
Cook pasta in boiling water until al dente. Reserve 1/2 cup pasta water; drain pasta and return to pot. Add cauliflower mixture, Parmesan, and parsley; toss to combine. Tossing pasta, gradually add enough pasta water to form a thin sauce that coats pasta. Serve topped with breadcrumbs and, if desired, more Parmesan.
Cauliflower tastes surprisingly nutty and sweet when roasted. Break it up into small florets to help it cook quickly. We like cavatappi, but any short pasta will work.