In a large pot of salted boiling water, cook the fettuccine according to package instructions until al dente. Reserve 1 cup of pasta water, then drain the pasta and set aside.
In a large skillet, heat the olive oil over medium heat. Add the minced garlic and sauté for about 1 minute until fragrant but not browned.
Stir in the pumpkin puree, heavy cream, ground nutmeg, and crushed red pepper flakes (if using). Mix well and let it simmer for 4–5 minutes to thicken slightly.
Gradually stir in the Parmesan cheese until melted and incorporated into the sauce. If the sauce is too thick, add some reserved pasta water, a little at a time, to reach your desired consistency.
Stir in the chopped sage and season with salt and pepper to taste. Add the cooked pasta to the sauce, tossing gently to coat the noodles evenly with the creamy pumpkin sauce.
Plate the pasta and garnish with additional sage leaves, freshly cracked pepper, and toasted pumpkin seeds for crunch if desired.