1. Stir together first 6 ingredients. Shape mixture into 4 (1/2-inch-thick) patties.
2. Heat a grill pan over medium-high heat. Coat grill pan with cooking spray. Add patties; cook 5 minutes on each side or until done.
3. Stir together cucumber, yogurt, mint, and 1/2 tsp. salt in a small bowl. Serve burgers on buns with cucumber sauce and desired toppings. Garnish, if desired.