Bring a pot of heavily salted water to a boil. Add fettucine and cook according to package directions. Drain and return to pot.
Meanwhile, heat oil in a medium skillet over medium-high heat. Add onions and let cook until soft. Add garlic and let cook a few more minutes. Remove from heat.
In a blender, combine onions and garlic, cashews, water, miso paste if desired (again, yes, add the miso), lemon juice, salt, and pepper. Process on high until very smooth, about 2 minutes.
Toss hot pasta with sauce until noodles are evenly coated. Adjust seasoning to taste. If sauce gets too thick, add a little water, 1 tablespoon at a time. Garnish with parsley and serve.
*If you are not using a high-powered blender, such as a Vitamix, soak cashews or almonds overnight or boil for 10 minutes and drain. This will soften them and ensure a silky smooth cream.
http://www.yourveganmom.com/your_vegan_mom/2012/05/chloes-fettucine-alfredo.html