In a large skillet, heat the oil. Add the onion, garlic, cumin and coriander. Cook over moderately high heat until the onion is softened. Add the stock, crushed tomatoes, tomato paste, oregano and bay leaves; bring to a boil. Add the chicken, cover partially and simmer until cooked, 35-40 minutes or until done.
When chicken is done, remove from the sauce and shred the meat. Return the meat to the sauce and simmer for 10 minutes. Discard the bay leaves and oregano sprig. Stir in vinegar and season with salt and pepper.
Heat the tortillas on a flat grill or in a hot pan. Fill the tortillas with the chicken mixture and a little sauce. Top with onions, avocados, cheese, cilantro and serve with a lime wedge.