In a large bowl, stir together the yogurt, lime juice, olive oil, orange zest, cumin, salt, pepper, saffron, coriander, garlic, and onions. Add the chicken thighs and toss to coat. Refrigerate for 4 hours.
Build a medium-hot fire in a charcoal grill, heat a gas grill to medium-high, or heat broiler to high. Skewer the chicken on 4 metal skewers, and skewer the tomatoes on 3 metal skewers. Grill the chicken, turning frequently, until cooked through and lightly charred, about 10 minutes. Grill the tomatoes, turning frequently for about 7 minutes, until soft and charred.
Sprinkle the chicken and tomatoes with sumac, and serve with limes and lavash or pita.