Sautee onion, spinach, stewed tomatoes in vegetable broth for about 6 minutes, add mushrooms for 4 more minutes. Keep just enough broth on until it evaporates then add more immediately. I add tons of spices here like onion/garlic powder, whatever you like.
stuff each mushroom with a spoon of the tomato, spinach mixture and drizzle mozzarella cheese on top.
bake on cookie sheet sprayed with cooking spray at 350 for about 15 minutes.