Grind the saffron threads in the palm of your hand, or in a mortar and pestle (preferred). Mix with 2 tablespoons of warm water in a large bowl. Let sit and allow saffron to bloom and draw out as much color as possible. It should be a deep ruby red. Set aside (I keep mine in the mortar).
Over a bowl, grate the onion completely. This will get juicy! When complete, gather grated onion in hands and squeeze excess water out or let drain in sieve. Place in another mixing bowl and discard onion water.
Add the yogurt, garlic, parsley, sumac, bloomed saffron, salt and pepper to the grated onion. Mix thoroughly to incorporate completely. Toss with the chicken to coat. Cover with plastic wrap and marinate 45 minutes.
Preheat your grill to 400°F. Clean grill and coat with cooking spray. Shake excess marinade from chicken and grill 4-6 minutes per side depending on thickness of chicken, covered. Discard marinade.
Serve with steamed rice and vegetables or salad of choice. In this meal I steamed some haricot vert with carrots. Then while hot, mixed in a bowl with some olive oil, 1/2 cup feta cheese and salt and freshly ground black pepper.