Combine yogurt and 1/4 cup tapenade in small bowl. Cover and refrigerate until ready to use.
Combine Ground Beef and remaining 1/4 cup tapenade in large bowl, mixing lightly but thoroughly. Lightly shape into four 1/2-inch thick patties.
Heat grill pan over medium heat until hot. Place patties in pan; cook 10 to 12 minutes until instant-read thermometer inserted horizontally into center registers 160°F, turning occasionally. Season with pepper, as desired. Remove; keep warm.
Add onion and zucchini to grill pan over medium heat; season with salt and pepper, as desired. Cook 5 to 6 minutes or until lightly browned and crisp-tender, turning occasionally.
Place 1 tablespoon cheese on bottom of each bun; top evenly with spinach leaves, onion, burger and zucchini. Spread 2 tablespoons yogurt mixture on cut side of each bun top. Close sandwiches.
Nutrition information per serving: 406 Calories; 171 kcal Calories from fat; 19 g Total Fat; 6 g Saturated Fat; 4 g Monounsaturated Fat; 74 mg Cholesterol; 655 mg Sodium; 30 g Total Carbohydrate; 4.8 g Dietary Fiber; 30 g Protein; 5.3 mg Iron; 7.3 mg Niacin; 0.6 mg Vitamin B6; 2.3 mcg Vitamin B12; 6.4 mg Zinc; 34 mcg Selenium;