Soak cashews overnight or boil water and then add the cashews to soak for 15-20 minutes to soften.
Dice the onion and sauté it in a pan over medium-high heat with a pinch of salt for 10-12 minutes, or until it begins to brown and soften.
Mince the garlic and add to the pan. Continue to cook for an additional 1-2 minutes.
Add the soaked and drained cashews, sautéed onions and garlic, lemon juice, dijon mustard, white miso paste, Kosher salt, black pepper, red pepper flakes and the water to a high-speed blender. Blend until smooth and creamy.
Note: You should be able to touch the sauce and rub it between two fingers and it feel completely smooth. If you feel any grit or graininess, keep blending.
Return the sauce back to the pan and bring to a simmer over medium heat until thickened, about 8-10 minutes.
Heavily salt a large pot of water and boil the pasta according to package instructions.
Add the cooked pasta to the cashew pasta sauce and toss to coat. Season with additional salt, if needed.
Garnish with lemon zest, red chili flakes, and parsley. Enjoy!