In a large skillet, melt 2 tablespoons butter. Add onions and garlic and cook over medium-heat until onions are softened and garlic is fragrant 2 - 3 minutes.
Scrape skillet ingredients into bowl of food processor. Add roasted red peppers. Cover and pulse until smooth.
Heat remaining butter in same skillet. Transfer red pepper puree from food processor to skillet. Add broth, salt, pepper and parsley. Stir to combine and cook until heated thru. Stir in cream, parmesan cheese and red pepper flakes. Cook until sauce begins to thicken.
Drain pasta and add to skillet. Toss to coat with sauce and add additional salt and pepper to taste.
Transfer to serving plate and garnish with additional parsley and parmesan cheese.