Remove stems from mushrooms; place mushroom caps in a large bowl. Add oil and toss to coat. Finely chop stems.
In a large skillet, saute the chopped mushrooms, walnuts and onion in butter until tender. Add garlic; cook 1 minute longer. Stir in the spinach, cheese, pepper and salt; cook and stir until cheese is melted. Stuff into mushroom caps.
Place on greased baking sheets. Bake at 375° for 12-14 minutes or until mushrooms are tender.