In a dish mix chicken broth, vinegar and rosemary and then in another dish put Chicken and pour the above mixture over it and refrigerate overnight. Now remove Chicken and pat it dry. Let the marinade remain in the dish. Heat oil in a heavy frying pan and brown Chicken in two batches. Remove Chicken from the pan; add onions and garlic. Saute over medium heat for 5 minutes, until onions are softened. Add marinade to pan and bring to a simmer. Add Chicken, paste with some of marinade, cover and simmer 25 minutes. Remove Chicken and keep warm. Add tomatoes, salt and pepper, simmer about 5 minutes, or until thickened. Pour sauce over Chicken.