1. Preheat broiler.
2. Drain spinach, and press between layers of paper towels until barely moist; set aside. Clean mushrooms with damp paper towels; remove stems. Finely chop stems; set aside.
3. Coat a large nonstick skillet with cooking spray; place over medium-high heat until hot. Add mushroom caps; cook, stirring constantly, 5 minutes. Drain. Place caps on rack of a broiler pan, stem sides up; set aside.
4. Combine spinach, chopped stems, yogurt, and next 8 ingredients; spoon evenly into mushroom caps. Sprinkle evenly with 1 tablespoon Parmesan cheese. Broil 5 1/2 inches from heat 5 minutes or until thoroughly heated. Serve immediately.