1. Combine first 6 ingredients and 1/8 teaspoon pepper in a large bowl, mixing gently. Divide turkey mixture into 6 equal portions, shaping each into a 1/4-inch-thick patty. Heat a grill pan over medium-high heat. Coat pan with cooking spray. Add patties to pan; cook 8 minutes on each side or until done.
2. Combine remaining 1/8 teaspoon pepper, yogurt, and next 4 ingredients (through mint) in a bowl, stirring well. Spread 1 1/2 tablespoons yogurt mixture on the bottom half of each bun; top each serving with 1 lettuce leaf, 1 onion slice, and 1 patty. Place top half of bun on each serving.