Bring a large pot of salted water to a boil. Cook pasta according to package directions until al dente. Drain, reserving ½ cup of pasta water.
Heat olive oil in a skillet over medium heat. Add chopped onion and cook 3–4 minutes until softened. Stir in garlic and cook another minute.
Transfer onions and garlic to a blender. Add roasted red peppers, cream (or yogurt), and Parmesan. Blend until smooth.
Pour sauce back into the skillet and simmer gently for 5–7 minutes. Season with salt, pepper, and red pepper flakes.
Add cooked pasta to the skillet, tossing to coat evenly. Add a splash of reserved pasta water to loosen the sauce if needed.
Serve warm, topped with fresh basil or parsley and extra Parmesan.