Olympian Burgers

Ingredients

• 1 pound Ground Beef (95% lean)
• 1/4 cup low-fat or regular plain Greek-style yogurt
• 1/2 cup olive tapenade, divided
• 1 small sweet onion, thinly sliced
• 1 small zucchini, thinly sliced
• 1/4 cup crumbled feta cheese
• 4 whole-grain hamburger buns, split, toasted
• Fresh spinach leaves