Preheat oven to 350°F. Coat each mushroom cap with nonstick cooking spray and place on aluminum foil-lined baking pan. Bake in 350°F oven 15 to 17 minutes or until mushrooms are fork tender.
Meanwhile, preheat large nonstick skillet over medium heat until hot. Add ground beef, cook 5 to 7 minutes, breaking into 1/2-inch crumbles and stirring occasionally. Add mushroom stems, onion, Greek seasoning, garlic and pepper; cook 3 to 5 minutes or until beef is thoroughly cooked and onion is translucent. Stir in spinach and 1/2 cup feta. Keep warm.
Evenly divide beef mixture among mushroom caps (about 1 cup each), mounding as needed. Top with tomatoes, remaining 1/4 cup feta and olives. Top with tzatziki, balsamic glaze, salad dressing, red peppers and cucumbers, as desired.
Nutrition information per serving: 319 Calories; 144 kcal Calories from fat; 16 g Total Fat; 8 g Saturated Fat; 5 g Monounsaturated Fat; 109 mg Cholesterol; 456 mg Sodium; 12 g Total Carbohydrate; 4.1 g Dietary Fiber; 35 g Protein; 4.8 mg Iron; 10.9 mg Niacin; 0.8 mg Vitamin B6; 3.3 mcg Vitamin B12; 8.2 mg Zinc; 45.3 mcg Selenium; 136.3 mg Choline;