Chicken breasts come in different sizes. If you have chicken breasts that are around a half pound each or more, you will want to slice them in half horizontally, so that the center thickness is around 1/2-inch to 3/4-inch thick. (We do not recommend pounding the chicken breasts, doing so will not result in the right texture/consistency for fajitas.)

Mix all the marinade ingredients together in a glass or plastic container. Add the chicken, mix well, cover and let marinate at room temperature for 1 hour. (You can marinate them in the refrigerator for up to 8 hours, but remove them an hour before cooking so that they can come closer to room temp.)

Remove the chicken from the marinade. Wipe off most of the marinade and sprinkle the chicken pieces with salt.

Heat a large cast iron frying pan on high heat for 1 to 2 minutes. Add a tablespoon of canola oil (or other high smoke point oil) to the pan. As soon as the oil begins to smoke, lay the chicken breast pieces in the pan. Depending on the size of the pan, and if you have had to cut the chicken breasts, you may have to work in batches. Let the chicken cook undisturbed for 2-3 minutes, until you have a good sear. Once seared well on one side, turn the pieces over and cook for another 2-3 minutes until well seared on the second side.

Once seared on the second side, remove to a cutting board and cover with aluminum foil to rest for 5 minutes. Here's a tip. Stack the seared chicken breasts and then cover them in foil. Together they will retain heat better as you cook the peppers and onions. If you want to test for doneness, cut into one piece with the tip of a sharp knife. It should be just done, if not, you can put it back in the hot pan for a minute or two.

While the chicken is resting, cook the onions and peppers. Add another tablespoon of oil to the frying pan. Heat on high. As soon as the oil is hot, add the onions and peppers to the pan. Use a metal spatula to scrape up some of the browned bits from the chicken and stir to coat the onions and peppers with the oil and brown bits. Spread the onions and peppers in an even layer in the pan. Let them cook undisturbed for 2 minutes. You want them to sear with some blackening. Stir the vegetables and continue to cook for another 2 minutes.