Preheat oven to 350 degrees.In a large pan, heat 2-3 tablespoons of water and water saute the chopped onion for 2 minutes. Add mushroom stems, garlic and thyme and continue to saute until onions and mushrooms are tender, about 3 minutes. Add mushroom caps to pan, along with vegetable broth, bring to a simmer and cook for 5 minutes.Remove mushroom caps from pan and place on a lightly oiled baking sheet. Add spinach to onion mixture remaining in pan and heat until wilted. Remove from heat and stir in almond butter, nutritional yeast and black pepper.Fill mushroom caps with spinach/onion mixture and bake for 15-20 minutes or until golden brown.Note: If desired, 1/2 cup of whole grain bread crumbs may be added to stuffing mixture (if gluten is not a problem).