Soak the cashews in hot water for at least 20 minutes, then drain.
Heat olive oil in a large skillet over medium heat. Add sliced onions, salt, and pepper.
Cook onions for 20-25 minutes, stirring often, until golden brown and caramelized. Add garlic and cook 1 minute more.
Meanwhile, cook pasta according to package instructions. Reserve 1/2 cup pasta water before draining.
In a blender, combine soaked cashews, water, nutritional yeast, lemon juice, garlic powder, and onion powder. Blend until completely smooth.
Add the cashew sauce to the caramelized onions in the skillet. Stir well and simmer for 2-3 minutes to heat through.
Add drained pasta to the skillet, tossing to coat in the sauce. Use reserved pasta water to thin sauce if needed.
Serve immediately, garnished with fresh parsley.