Prepare pasta according to package directions.
Soak raw cashews in warm water for a minimum of 10 minutes.
While water pasta water is coming to a boil, and cashews are soaking dice and sauté both of the onions in a 1/4 cup of vegetable stock on medium heat for about 3 minutes or until translucent.
Take half of the sautéed onions, and add to the blender for the sauce. Keep the remaining half of the onions in the pan.
Dice mushrooms, and add to the pan with the onions. Sauté until browned about 5 minutes. Add spinach. Cover with lid, and allow steam to wilt the spinach.
Drain soaked cashews. Add to the blender with the onions. Add garlic, almond milk, vegetable stock, nutritional yeast, salt, pepper, and optional lemon juice. Blend until smooth. Transfer to a small pot. Heat on medium until sauce is warm and thickens. Set aside.
Assemble the pasta. In the pan with mushrooms and spinach, add the cooked and drained pasta. Pour cashew cream sauce over top, and stir until sauce is well combined. Serve with a wedge of lemon, or a sprinkle of vegan parmesan.