To prepare the kebabs, skewer the lamb pieces alternating with the onions into 6 large metal skewers. Place the kebabs in a large baking dish. In a small bowl, whisk together the garlic, orange zest and juice, mint, and oil. Pour the marinade over the kebabs and refrigerate for at least 30 minutes, or up to 2 hours.
While the lamb is marinating, prepare the yogurt sauce. Whisk together the yogurt, garlic, cumin, and chile powder in a bowl. Season, to taste, with salt and pepper. Refrigerate until you are ready to serve.
To grill the kebabs, oil the grill and prepare a hot fire. If you are using a gas grill, preheat on high. If you are using a charcoal grill, let the coals burn until they are covered with gray ash. The fire is hot when you can hold your hand over the grill for no longer than 5 seconds. Remove the lamb from the marinade and drain very well. Place the skewers on the grill and grill, turning once or twice, for 6 to 8 minutes for medium doneness.
Place the skewers on a serving platter, and garnish with mint sprigs. Serve warm, with the sauce on the side.