Preheat oven to 450 degrees Fahrenheit. Spray portobello mushrooms with olive oil and place in a pan with stem up.

Sprinkle salt all over the mushrooms and bake for 15 minutes. At the end of the baking time, remove mushrooms from the oven and drain mushroom liquid.

Spray oil on a sauté pan and sauté onion on medium heat for 15 minutes. Add salt, garlic powder, spinach, nutritional yeast, mozzarella cheese if you are using it, and let it wilt for four minutes.

Place onion-spinach mixture on top of mushrooms.