Preheat oven to 200 degrees C. Heat the oil in an ovenproof dish or frypan and cook the onion, garlic and chicken till brown, about 10-15 minutes. Transfer to a casserole dish. Take the rosemary off the sprigs and sprinkle over chicken. Add potatoes and tomatoes and pour over chicken stock.
Bake, uncovered, for an hour or until cooked. Stir in olives just before serving.