1. Place grated onion in a sieve over a large bowl for 30 minutes, pressing occasionally to squeeze out excess liquid. Discard liquid.
2. Prepare grill to medium-high heat.
3. Combine onion, breadcrumbs, and next 8 ingredients (through lamb) in a large bowl; stir until well blended. Divide meat mixture into 40 equal portions, shaping each into a 3 x 1–inch rectangle. Thread 4 pieces onto each of 10 (12-inch) skewers.
4. Heat a small skillet over medium heat. Add saffron to pan; cook for 30 seconds or until fragrant, stirring constantly. Crush saffron in a small bowl; stir in oil.
5. Place kebabs on a grill rack coated with cooking spray; grill 8 minutes or until done, turning after 5 minutes. Place kebabs on a platter. Brush oil mixture evenly over kebabs. Sprinkle kebabs evenly with sumac. Combine yogurt and remaining ingredients in a small bowl; stir until well blended. Serve kebabs with yogurt sauce.