In non-metal bowl, place chicken, yogurt, onions, cumin, rosemary, salt and pepper ( to taste) mix well. Cover and refrigerate 3-4 hours.
Preheat oven to 375 F. Place chicken in yogurt mixture in shallow baking pan. Bake 20 minutes.
Sauce: in bowl, combine first 3 sauce ingredients; mix well. Add pinch of salt.
Remove chicken from oven and drizzle 1/2 tsp. olive oil over each breast. Place pan under broiler and cook 5 minutes until onions begin to blacken and chicken is cooked to desired doneness.
Serve with sauce. Garnish with fresh cilantro if desired.