1. If using fresh peppers, preheat the oven to 450°F (230°C). Roast the red bell peppers for about 20 minutes, turning occasionally, until tender with dark roasted patches. Cover with foil and steam for 10 minutes. Remove the skins, stems, and seeds. If using jarred roasted peppers, drain them thoroughly.

2. Bring a large pot of generously salted water to a boil. Cook the fettuccine or penne according to package directions until al dente. Reserve about 1 cup of pasta water before draining. Do not rinse the pasta.

3. Heat the olive oil and butter in a large skillet over medium heat. Add the chopped onion and cook for 3–4 minutes until soft and translucent. Add the minced garlic and cook for about 1 minute until fragrant.

4. Transfer the cooked onion and garlic to a blender. Add the roasted red peppers, heavy cream, and Parmesan. Blend until smooth. Add a small splash of reserved pasta water if needed to help the mixture blend.

5. Pour the roasted red pepper sauce back into the skillet. Warm over low heat for 3–4 minutes. Season with salt, black pepper, and optional red pepper flakes.

6. Add the drained pasta to the skillet. Toss until the pasta is evenly coated. Add reserved pasta water a little at a time until the sauce reaches a silky, glossy consistency.

7. Remove from the heat. Divide the pasta among serving bowls and finish with grated Parmesan, chopped fresh basil, black pepper, and optional red pepper flakes. Serve immediately.