Heat the grill to a high heat (about 400 degrees F), and make sure grates are oiled and clean. Pound chicken breast to an even thickness by placing between two sheets of parchment paper or plastic wrap and pound with a heavy rolling pin or meat mallet.
Combine the juice of 2 limes, olive oil, garlic, chili powder, cumin and salt in a large zip close bag and shake to combine. Add in the chicken breasts and let marinade for at least 15 minutes.Grill the chicken over the hot grill for 3 to 5 minutes per side, until internal temperature reaches 165 degrees F. Let rest for 5 minutes under foil before slicing into strips.
Meanwhile, heat a large cast iron skillet or non-stick skillet over high heat until it's smokin' hot. Toss in the peppers and onions (no oil is necessary), and let cook, stirring occasionally, until the peppers and onions are charred and tender-crisp. Add in the oregano and cook until fragrant.
Serve the grilled chicken with the charred peppers and onions along with warm tortillas and all the toppings you desire. Enjoy!
Chicken can be marinated for 15 minutes, up to 24 hours. Don't marinate over that amount of time.