If you weren't able to or forgot to soak your cashews overnight, don't worry there is a shortcut. Just put the raw cashews in a saucepan, cover them with water, and bring them to a boil. Once the cashews come to boil, cover the saucepan with a lid, remove from the heat, and let them sit for 15 minutes to soften up. Then drain and set aside.
Boil the cauliflower florets in salted water until good and done. Then drain, but do not discard the cooking water. You will need it in a later step of the recipe.
Get your salted pasta water prepped by turning it on to come to a boil. You don't want to add your fettuccine just yet, but having it ready will save you time.
Add the cauliflower, cashews, nutritional yeast flakes, garlic powder, onion powder, and ½ cup of the cauliflower cooking water to your blender or food processor. Puree to a smooth texture. Add more of the cooking water as needed to reach a consistency that is smooth like sauce, but not thin like water. Creamy like a traditional white sauce.
Start cooking your noodles now. Don't forget to set a timer.
Now, in a cold saute pan add the grapeseed oil and garlic. Turn the heat on to medium to cook the garlic for about about 2 minutes until you can smell the aroma. You don't want to turn the heat on until after you add the oil and garlic because you aren't trying to brown the garlic. You just want it to soften up and sweat. Too high of a heat will burn delicate garlic.
Add the vegan butter to the saute pan with the garlic. Once the vegan butter is melted add the mushrooms with a sprinkle of salt and cook at medium-high for 2-3 minutes. Move the mushrooms around the pan until they are evenly cooked.
When the mushrooms are done add the cauliflower mixture to the saute pan. Cook for about 2-3 minutes on medium to let the flavors meld together.
Taste your sauce and season with salt to your liking.
When the pasta is ready, drain and add the sauce to the hot noodles. Taste again to season as needed.
To serve, top with vegan parmesan cheese, fresh chopped parsley, and ground black pepper.