Preheat oven to 425°F. Toss cauliflower florets with 2 tablespoons olive oil, salt, and pepper. Spread on a baking sheet and roast until golden and tender, about 25 minutes.
Bring a large pot of salted water to a boil. Add pasta and cook according to package instructions until al dente. Reserve 1 cup of pasta water, then drain.
In the same pot, heat 1 tablespoon olive oil over medium heat. Add garlic and red pepper flakes, sauté for 1 minute. Add the roasted cauliflower, cooked pasta, lemon zest, lemon juice, and half of the reserved pasta water.