Slice the chicken breasts into thin strips, about the size of your finger. Combine the lime juice, olive oil, garlic, salt, cumin, chili powder and red pepper flakes in a bowl and whisk to combine. Add the chicken and marinate for 1 hour.
Heat a large cast iron skillet over high heat. Add a tablespoon of olive oil. Add the chicken mixture, marinade included, and sauté until fully cooked. Using a pair of tongs, remove the chicken from the skillet and add the vegetables. Sauté until tender and charred. Add the chicken back into the cast iron skillet and toss to combine.
Stuff the tortillas with the chicken fajita mixture and top with salsa, avocado, sour cream and the shredded Monterey jack cheese. Garnish with cilantro as needed and serve.