Preheat the oven to 425°F. Bring a large pot of salted water to a boil.
Toss the tomatoes, chopped onions and garlic cloves with the olive oil, ranch seasoning mix and pepper. Spread out onto a sheet pan. Roast for about 20 minutes, shaking occasionally, until the tomatoes are blistered and the onions are softened.
While the tomatoes are roasting, cook pasta according to package directions until al dente. Drain, reserving ¾ cup of the pasta water. Set both aside.
When tomatoes are done, transfer to a blender, reserving about ¾ of the tomatoes, and puree until smooth. Add the Parmesan Ranch dressing, parmesan cheese, and enough pasta water to thin to desired consistency.
Toss the sauce with the pasta and remaining roasted tomatoes. Garnish with additional parmesan cheese. Serve warm.