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
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 degrees 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.