1. Prepare grill.
2. Remove brown gills from undersides of mushrooms using a spoon; discard gills. Set mushroom caps aside.
3. Heat 1 teaspoon oil in a large nonstick skillet over medium-high heat. Add garlic, and onion; sauté 2 minutes. Add oregano and spinach; saut 1 minute or until spinach wilts.
4. Transfer spinach mixture to a medium bowl; stir in remaining 1 teaspoon oil, cheese, panko, vinegar, and pepper. Divide filling evenly among mushrooms, spooning onto gill sides.
5. Grill 7 minutes.
Serve with: Grilled Tomato Salad