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