1 medium cauliflower (about 2 pounds)
1⁄4 cup extra virgin olive oil
1 medium white onion, cut into 1⁄4-inch dice
3 garlic cloves, smashed and peeled
Maldon or other flaky sea salt
11⁄2 to 2 teaspoons hot red pepper flakes
6 tablespoons unsalted butter, cut into 6 pieces
1 pound pennette
3⁄4 cup freshly grated Parmigiano-Reggiano, plus extra for serving
1⁄2 cup coarse fresh breadcrumbs, fried in olive oil until golden brown
11⁄2 teaspoons minced fresh rosemary
Halve the cauliflower. Cut off the leaves and reserve them. Cut out the core and reserve it. Cut the cauliflower into small bite-sized florets, reserving the stalks. Chop the core, stalks, and leaves.
Combine the oil, onion, garlic, and cauliflower leaves, stalks, and core in a large pot, season with Maldon salt, and cook over medium heat, stirring frequently, until the leaves are just beginning to wilt, about 3 minutes. Reduce the heat to low and cook, stirring frequently, until the cauliflower leaves are just tender, 18 to 20 minutes.
Add the cauliflower florets, red pepper flakes, and 1 cup water and bring to a simmer over medium-high heat, then reduce the heat to a gentle simmer, cover, and cook, stirring occasionally, until the cauliflower is very soft and almost falling apart, 22 to 25 minutes. Add the butter, stirring gently until it melts, then season well with Maldon salt and remove from the heat. (The cauliflower ragu can be prepared up to 3 days ahead. Let cool, then cover and refrigerate; reheat in a large pot over medium-low heat before adding the pasta.)
Bring 6 quarts water to a boil in a large pot and add 3 tablespoons Kosher salt. Drop in the pasta and cook until just al dente.
Drain the pasta, reserving about 2/3 cup of the pasta water. Add the pasta and 1/3 cup of the reserved pasta water to the cauliflower ragu and stir and toss over medium heat until the pasta is well coated (add a splash or two more of the reserved pasta water if necessary to loosen the sauce. Stir in the cheese.
Transfer the pasta to a serving bowl, sprinkle with the bread crumbs and rosemary, and serve, with additional grated cheese on the side.