Trim any fat, silverskin, or cartilage from the breasts. Wash the chicken in half water and half vinegar if you want to. Drain.Mix all marinade ingredients in a bowl.Place chicken and marinade in a 2.5-gallon ziplock bag.Move the chicken around by massaging the bag so that all the meat is coated with the marinade. Lay the bag flat on the bottom shelf of the refrigerator, making sure no ready-to-eat foods are below it. Refrigerate for 24 hoursFlip the bag every 6-12 hours to ensure an even marinade.Preheat grill to mediumPreheat the oven to 350.Grill chicken for 3-4 minutes on each side, turning 90 degrees after about 1-2 minutes to get an excellent grill mark. Do this on the other side as well.When the grill marks are nice and brown, pull the chicken off the grill, cover it with foil, and let it rest for 30 minutes. Cook your chicken until it reaches an internal temperature of 165 F in the thickest part. If it doesn't reach this temperature by the instruction so far, place the chicken in the oven or back on the grill until the temperature is reached.
Slice the whole breasts of chicken into 1/4 inch strips across the grain. You will see the striations. Simply cut across the grain instead of in their direction. This will produce a more tender and enjoyable bite.Peel and slice the onions in half and then into 1/4" slices.Wash, core, seed the peppers, and slice into 1/4" slices.In Skillet 1Get your skillet hot on medium-high heat, then add the first portion of beef fat or avocado oil. Toss the onions, peppers, and salt in and saute until desired tenderness with the desired char. Turn off the heat and set aside.In Skillet 2Heat cast iron skillet on med/high to high heat to get it hot. Add the second portion of beef fat or avocado oil to the skillet, and spread around with a spatula or by swirling. CAUTION: This oil will get hot fast, so don't splash any out, or you can hurt yourself or someone else.Add the chicken slices and toss to coat with oil using a set of tongs. Keep tossing and stirring until all the chicken is coated with oil and hot.Turn off the heat, slide meat to one side, add peppers and onions from the other pan, and serve.If you want the restaurant effect of smoking fajitas on a hot skillet, you will need to get a 3rd skillet screaming hot and transfer the sauteed chicken, peppers, and onions to it. You will get the desired effect, but it is dangerous and impractical at home. Eating it is more fun than picking it up off the floor after that hot pan has burned you.Be careful and enjoy.