Place the cashews in a small bowl or cup and pour some boiling water over to cover. Set aside for at least 15 minutes to soak (preferably longer) and drain.
Spiralize the zucchini using your favorite spiralizer such as this one from Amazon.
In the meantime, bring a large pot of water to a boil, add 1 -2 tablespoons of salt and cook the pasta al dente according to packaging instructions. Drain the pasta reserving some cooking liquid (a cup or so) and set aside.
In a large skillet heat olive oil and add sliced mushrooms and chopped onion. Cook on medium heat until browned, stirring often. Add minced garlic at the end and cook together stirring until fragrant.
While the mushrooms and onions are cooking, place the soaked cashews in a standup blender, add nutritional yeast, garlic and ⅔ cup water. Blend until smooth.
Pour the cashew cream into the mushroom, onion and garlic mixture, stir and bring to a gentle boil. Turn the heat down, add spiralized zucchini and toss together. Cook zucchini in the sauce for about 2 minutes.
Once the zucchini is softened, toss the spaghetti into the sauce and stir together. Loosen the sauce with some liquid you reserved from cooking the pasta until you reach the desired consistency, though no more than ¼ cup at a time or it might turn too watery.
Serve immediately sprinkled with chopped herbs and red pepper flakes (if using) and enjoy!