1. Heat oven to 450° F. Cook the pasta according to the package directions. Drain the pasta and return it to the pot, reserving 1 cup of the cooking water.
2. On a large baking sheet, toss the Brussels sprouts and onion with the rosemary, 2 tablespoons of the oil, and salt and pepper, to taste. Roast, tossing the vegetables once, until golden brown and tender, 15 to 20 minutes.
3. To the pasta, add the roasted vegetables, garlic, ½ cup of the reserved cooking water, Romano cheese, and the remaining 2 tablespoons of oil. Toss to combine (add more cooking water if the pasta seems dry).