Combine all the ingredients for the marinade in a small mixing bowl and whisk well.
Take out about 1/3 cup of the marinade and set it aside to cook vegetables.
Place chicken breasts into a zip-top bag and add the marinade. Get as much air out of the bag as you can, seal well, and gently shake it up to coat the chicken. Place the bag in a bowl or on a plate and into the refrigerator to marinate.
Marinate it for 2-4 hours.
Cooking Chicken Fajitas:
Slice onions and peppers and preheat a large cast iron skillet over medium heat.
Add a couple tablespoons of oil to the pan and add the vegetables. Stir vegetables well and season with a little bit of salt and pepper. Cook them until softened.
When vegetables are softened, pour in most of the reserved marinade all over the vegetables and mix well. Leave a couple of tablespoons of the marinade to finish the dish.
Finish cooking the vegetables until completely soft and start to golden. Take everything out of the pan and set it aside.
Take the chicken out of the marinade and into the same skillet. Add a little bit of the marinade from the bag to the chicken and discard the rest. Cook chicken for 6-8 minutes on each side, until internal temperature reaches 165°.
Take chicken out of the pan and take the pan off heat until ready to return sliced chicken to it.
Slice chicken thinly. Place the pan back on the heat and add sliced chicken and vegetables. Mix everything well and pour in remaining reserved marinade.
Cook for just a couple more minutes and serve on warm tortillas. Note: you can warm up the tortillas in another pan while chicken is cooking!
Notes
*Gluten free note: If y0u need to keep these fajitas gluten free, use corn tortillas and double check the packaging for the gluten free notice.
Additional toppings: Any toppings that you love on your tacos would be delicious on fajitas as well. Try some fresh avocados, jalapeño peppers, sour cream, guacamole, salsa, corn salsa, cheese, and so much more.