Preheat oven to 400°. Remove stems from mushrooms and set caps aside; discard stems or save for another use. In a small skillet, heat olive oil over medium heat. Add spinach; saute until wilted. Add garlic; cook 1 minute longer.
Combine spinach mixture with remaining ingredients. Stuff into mushroom caps. Place in a 15x10x1-in. baking pan coated with cooking spray. Bake, uncovered, until mushrooms are tender, 12-15 minutes. Serve warm.