Preheat a large pot or sauté pan over medium-low heat. Add the tahini/cashew butter to the pan, then the garlic. Stir to combine, mashing the garlic into the pan with the back of a spoon. After a minute, add a splash or two of water to thin the mixture. Stir and cook for another 2 to 3 minutes. Transfer the garlic mixture to a high speed blender and set aside.
Spread the cauliflower evenly in the same pan used to sauté the garlic. Sprinkle with a pinch of salt and add water for steaming, about half inch deep. Steam the cauliflower until tender, 5 to 7 minutes. Transfer the cauliflower to the blender and discard the cooking water.
To the blender add the remaining sauce ingredients: 1 cup water, nuts, lemon juice, nutritional yeast, salt, onion powder, mustard powder, and pepper. Blend until completely smooth. Taste and adjust seasonings.
Cook pasta according to package directions. Drain pasta in a colander, and pour the alfredo sauce into the pot. Cook over medium-low heat for about 2 minutes, just until heated through. Return the pasta to the pot and stir to coat.
Garnish individual servings with fresh parsley or basil, vegan parm, and extra black pepper, if desired.
Store leftovers in the refrigerator up to 4 days.If you don't have a high speed blender, substitute 3 Tbsp raw cashew butter for the cashews/walnuts.