Heat the oil in a 12-inch skillet. Add the sliced peppers and onions and season with salt and pepper. Cook, stirring occasionally, until softened and well-browned, about 5 to 7 minutes. Transfer the mixture to a bowl or plate and cover with foil, store in warm place.
Combine the orange juice, 1/2 cup of the of the cilantro, the chipotles, Worcestershire, and garlic in a measuring cup, then add to the skillet. Season the chicken with salt and pepper and add to the skillet. Bring the liquid to a simmer over medium-low heat, about 5 minutes. Flip the chicken over, cover, and cook until the chicken registers at least 165 degrees F on an instant-read thermometer. This may take 10 - 12 minutes, or longer depending on the thickness of your chicken. You will need to keep an eye on the liquid during this step, adding more chicken broth as necessary to avoid burning the sauce due to evaporation.
Transfer the chicken a plate and shred. If necessary, simmer the sauce until reduced to 1/4 cup. Return chicken to pan. Off heat, stir in mustard, tomatoes, green onions, and remaining 1/4 cup cilantro. Season with salt and pepper to taste.
Serve the chicken mixture with the peppers/onions and warm tortillas (or rice) with toppings if desired.