In a blender, mix together the lime juice, olive oil, balsamic vinegar, Worcestershire sauce, garlic, cumin, and chiles de arbol. Blend until smooth, and add salt and black pepper to taste. Pour the marinade over the chicken, and marinade refrigerated for at least 1 hour.
To make the fajitas, drain the chicken from the marinade. In a large, heavy skillet, heat up 1 tablespoon of oil on medium heat. Add the chicken, and cook covered for about 5 minutes per side, or until the internal temperature is 160°F. Remove the chicken from the skillet, and allow to rest for 10 minutes.
Meanwhile, add the remaining 1 tablespoon of oil to the skillet, and cook the bell pepper and onion slices on medium heat until tender and soft, about 7-10 minutes. Add salt to taste. While the peppers and onions are cooking, heat up the flour tortillas by either cooking each one over a burner or in a hot, dry skillet for about 15 seconds per side or until they puff.
Slice the chicken and serve with the bell peppers, onions, warm flour tortillas, guacamole, pico de gallo and sour cream, so people can make their own tacos.
If you can’t find chiles de arbol just use serrano chiles. The taste will be a little brighter but still fiery.