In a large bowl, combine the yogurt, tomatoes, garlic, ginger, chilies, and spices. Marinate the chicken in this mixture for an hour or two. In a heavy skillet, heat the oil over medium heat, and saute the onions until soft. Add the chicken with its marinade, and cook, covered, over medium heat for about 45 minutes, or bake it in the oven at 350 degrees F until the chicken is tender, about 1 hour. Add salt and pepper to taste. Serve with rice.