Fill a medium saucepan half full with water and place on stove over high heat. Add cauliflower, cashews, garlic, and onion. Boil for 10 minutes until cauliflower is VERY soft - should crumble easily when pierced with a fork.
Drain water and transfer the cauliflower, cashews, garlic and onion to a high speed blender. Meanwhile, fill the same medium sauce pan with water and cook pasta according to package instructions while you finish making sauce.
Drain cooked pasta, reserving ⅔ cup of pasta water and set aside. Return the cooked pasta to the pan.
Add the remaining ingredients along with ½ cup of pasta water to the blender and blend on high until super smooth and creamy. Add more water if needed. Season to taste.
Pour blended Alfredo sauce over the noodles and stir to coat, adding pasta water little by little if needed.
Sprinkle with parsley and serve hot, topped with fresh cracked black pepper, reserving a little extra parsley for a finishing garnish.
