Bring a large pot of lightly salted water to a boil. Add fettuccini and cook to desired doneness. While this occurs, heat the oil in a skillet set over medium-high heat. Season scallops and prawns with salt and pepper and then add to the pan and cook just through, about 3-4 minutes. Remove from the pan and reserve. Add remaining oil to the skillet, and then add the onions, garlic, chili flakes and asparagus and cook 3-4 minutes. Add 3/4 of a cup of the pasta cooking water to the pan and bring to a simmer. Drain pasta and add it, the reserve seafood, roasted red peppers, and the parsley or basil to the skillet. Season with salt and pepper and toss well. Serve pasta with freshly grated Parmesan cheese and warm crusty bread.