Once the squash mixture is done, remove it from heat and transfer the contents of the skillet to a high-speed blender. Puree the mixture until smooth (you could also puree it in using an immersion blender). Combine the cooked fettuccine and butternut squash puree back in the skillet (if sauce is too thick, add the reserved pasta water, 1/4 cup at a time, until sauce reaches your desired consistency). Cook over medium heat, tossing and adding more pasta water as needed, until the sauce coats the pasta, about 2 minutes. Season with more salt and pepper if necessary.