Preheat oven to 450ºF. Place cauliflower, carrots and onions on a parchment-lined rimmed baking sheet. Drizzle with 2 Tbsp olive oil and toss to coat. Season with salt and pepper. Roast for 10 minutes, stir, and continue roasting until golden in spots; about 10 minutes.
Boil pasta as per package directions. Drain, making sure to reserve 1/2 cup of pasta water. Return pasta to pot and add sun dried tomatoes, raisins, parmesan and remaining oil. Toss in roasted vegetables and reserved pasta water, season to taste.
Serve sprinkled with additional parmesan and scallions.