Preheat oven to 425°F. Line a sheet pan with foil or parchment paper.
Cut cauliflower into bite-sized florets and place in a bowl. Drizzle with half of the olive oil to coat. Add salt and pepper, half of the lemon zest, 2/3 of the minced garlic and chili flakes, then toss well and place on the lined sheet pan.
Roast until tender, golden and caramelized in places, about 20-25 minutes.
Cook the pasta according to the package directions. Reserve 1 cup of the pasta cooking water, then drain the pasta and place in a large serving bowl.
Drizzle pasta with the remaining olive oil, then add the remaining garlic, chopped parsley, lemon zest, walnuts and roasted cauliflower. Add hot pasta water to make a sauce. Toss everything together and serve.