Cook the fettuccine according to the packet’s instructions. Drain, reserving 2 cups of the cooking water, and set aside.
Place all the sauce ingredients into a food processor and process until a thick paste-like consistency is reached. With the motor still running, slowly add the pasta cooking water until a creamy sauce consistency is reached.
Pour 2 cups of the sauce into a large pan along with the parmesan and heat slowly until it begins to bubble. Stir in the pasta until combined. Divide among bowls and top with extra parmesan and sage leaves.
The remaining sauce will keep in the fridge for up to a week. (see note)
This pasta dries out when when it cools. If you’re making this with the intention of saving some for leftovers, I would add extra sauce when reheating.