Bring a large pot of salted water to a boil. Add the fettuccine and cook until al dente. Drain, reserving 1 cup (237 ml) of the cooking water.
While the pasta is cooking, make the sauce. In a high-powered blender or food processor, combine the cashews, lemon juice, non-dairy milk, salt and pepper, and blend until very smooth. Depending on the strength of your blender, this could take anywhere from 1–3 minutes.
Add in the dried sage, freshly ground nutmeg and canned pumpkin. Blend for another minute, until combined, then season to taste with more salt and pepper, if needed. Just before serving, toss the sauce with the hot noodles. If sauce is too thick, thin with reserved pasta water.