Bring a large pot of water to a boil. Add pasta and cook until al dente - approximately 10-12 minutes. Drain pasta, reserving 1 cup of pasta water, and set aside.
Place the pot back on the stove over medium heat. Add butter, garlic, onions and pepper to the pan and saute until onions are soft - approximately 5-7 minutes. Add roasted red peppers and red pepper flakes to the pan and saute for an additional 2 minutes.
Remove pan from heat and add cooked pasta, basil and salt to the pan. Slowly add pasta water to the pan until desired consistency is reached.
Taste and adjust seasoning as needed.