Set oven to broil. Place red bell pepper quarters skin side up on a foil lined baking sheet. Place under the broiler for about 10-15 minutes or until skins are blackened all around. Remove from the oven and place peppers in a sealed ziploc bag for 10 minutes. Remove the skins with your fingers (they should slip right off). Roughly chop the softened peppers. Set aside.
Cook pasta in heavily salted water to al dente, according to package directions. Drain and set aside.
Meanwhile, in a 5 quart pot, heat olive oil over medium heat and cook onions until soft; about 5 minutes. (If using raw garlic, add it during the last minute of cooking time).
Carefully transfer roasted peppers, garlic and onions to a blender or food processor and puree.
Add butter to the pot and melt over medium heat. Pour the red pepper puree into the pot. Add broth, heavy cream, nonfat milk (or any combination of cream and milk) and a pinch of salt and pepper. Stir to combine, taste and add a pinch of salt and pepper as desired.
Remove pan from heat and stir in Parmesan cheese and parsley. Add half of the pasta and gently toss to coat. Add remaining pasta and toss.
Garnish with grated Parmesan and chopped parsley and serve immediately.