In a large resealable plastic bag (or a large mixing bowl) add all the ingredients except the coarse sea salt and black pepper and toss to combine.
Cover and marinate in the fridge for 2 hours up to 8 hours.
Remove the meat from the marinade, transfer to a clean large surface or cutting board and pat dry with paper towels. Generously season both sides of the steak with coarse sea salt and black pepper.
In a large skillet over medium-high heat, add 1 tablespoon of cooking oil. Add the steak to the skillet and cook on each side for about 3-5 minutes, depending on how thick the steak is and how "done" you want it to be. I recommend using a meat thermometer for this step if you have one.
Remove the steak from the skillet, transfer to a cutting board to let it rest and lightly cover with aluminum foil.
While the meat is resting, add the remaining 1 1/2 tablespoons of cooking oil to the skillet. Add the onions, peppers and a pinch of salt. Cook, stirring occasionally, for about 8 minutes, until the veggies are soft.
Uncover the steak and slice it against the grain and at an angle into thin slices. Serve along with the fajita veggies in tortillas, on a salad or in lettuce cups and enjoy!
Notes
Use coarse sea salt after marinating the steak to help further tenderize and break down the fibers of the meat.Cut your steak against the grain and as thinly as possibly to prevent it from being chewy.If you're looking to add a little more flavor to the peppers and onions, you could reserve a tablespoon or two of the marinade before you use it for the meat, and add it to the skillet when you're ready to cook the veggies.