How to Make It
Heat oven to 450° F. Cook the pasta according to the package directions. Reserve 1 cup of the cooking water; drain the pasta and return it to the pot.
Meanwhile, on 2 large rimmed baking sheets, toss the cauliflower, Brussels sprouts, and onion with the thyme, 2 tablespoons of the oil, and ½ teaspoon each salt and pepper. Roast, tossing the vegetables once and rotating the sheets halfway through, until golden brown and tender, 15 to 20 minutes.
Add the vegetables, pecorino, ½ cup of the reserved cooking water, and the remaining 2 tablespoons of oil to the pasta and toss to combine (add more cooking water if the pasta seems dry). Serve sprinkled with additional pecorino.