1. Combine the marinade ingredients in a small bowl and stir to combine.

2. Add the chicken breasts to a large, non-reactive bowl and pour the marinade on top. Turn the breasts to ensure all pieces are equally coated. Allow to marinate at room temperature up to one hour or cover and place in the refrigerator overnight.

3. To prepare the fajitas, heat 2 tablespoons olive oil in a large cast iron skillet over medium heat. Remove chicken breasts from marinade and discard remaining liquid. Place chicken breasts in skillet and cook for 7-8 minutes before turning. Continue cooking another 6-8 minutes, or until chicken is cooked through. Remove from heat and keep warm. 

Tip: An instant-read thermometer (like this one -https://amzn.to/40eqEbH should read 160 degrees when inserted into the thickest part of the chicken. The heat will continue to rise another 5 degrees while the meat rests. 

4. Increase heat to medium-high and add remaining olive oil to skillet. Add the sliced peppers and onion and cook, stirring frequently, until crisp tender and nicely browned, approximately 6-7 minutes.

5. To serve, cut the chicken breasts into thin slices and divide among cast iron fajita platters, if using, (see tip above) or individual serving plates. Serve immediately with peppers, onions, sliced avocado, lime wedges, and fresh cilantro, if desired, on the side.