Preheat the grill or grill pan to medium high heat. In a large bowl, combine ground lamb, onion, bread crumbs, mint, eggs, garlic, onion, cumin, salt and black pepper. Gently mix with your hands until all ingredients are well combined. Form into 10 equal-sized patties and place onto a parchment-lined baking sheet. Place lamb burgers on the grill and cook for 4-5 minutes per side, or until meat has reached the desired doneness. Serve warm burger on a toasted bun with tzatziki sauce.