In a large pan, heat 2-3 tablespoons of water and water sauté chopped onion for 2 minutes, add mushroom stems, garlic and thyme and continue to sauté until mushrooms are tender, about 3 minutes.
Add mushroom caps to the pan, along with stock, bring to a simmer, and cook for 5 minutes.
Remove mushroom caps from the 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 peanut butter and black pepper.
Fill mushroom caps with spinach/onion mixture and bake for 15 to 20 minutes or until golden brown.
