Preheat oven to 350 degrees. Coat each mushroom cap with nonstick cooking spray and place on aluminum foil-lined baking pan. Bake in 350-degree oven 15–17 minutes or until mushrooms are fork tender. Meanwhile, preheat large nonstick skillet over medium heat until hot. Add ground beef, cook 5–7 minutes, breaking into 1/2-inch crumbles and stirring occasionally. Add chopped mushroom stems, onion, Greek seasoning, garlic, and pepper; cook 3–5 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.