In the blender, add soaked cashews, water, garlic, lemon juice, avocado, nutritional yeast, and kale; blend until smooth, adding water as needed, and season with salt and pepper.
Combine cooked pasta, reserved water, mushroom onion mixture, and sauce in a large bowl; toss to coat evenly, then serve topped with parsley and red pepper.
Notes
For extra creaminess, blend a little more avocado into the sauce.