Build your marinade first. In a medium bowl or large zip-lock bag, combine 2 tablespoons of the olive oil, the lime juice, orange juice, minced garlic, smoked paprika, cumin, chili powder, onion powder, kosher salt, black pepper, and cayenne if you are using it. Whisk everything together until it forms a cohesive, deeply aromatic mixture. This is the flavor foundation of your entire cook, so taste it and make sure it has that bold, tangy, smoky punch you are looking for.

Add the boneless skinless chicken thighs to the marinade and toss to coat every surface completely. Seal the bag or cover the dish with plastic wrap and refrigerate for a minimum of 2 hours. If you have the time, marinate overnight for the deepest flavor penetration. Do not go beyond 24 hours or the citrus acids will start to break down the texture of the meat in an unpleasant way.

About 30 minutes before you are ready to cook, pull the chicken out of the refrigerator and let it come closer to room temperature. Cold meat hitting hot grates causes uneven cooking and can tighten up the muscle fibers fast. While the chicken is resting on the counter, fire up the grill. For a charcoal setup, use a chimney starter to get a full load of coals going until they are covered in gray ash, then pour them onto one side of the grill for a two-zone configuration. Your direct heat zone should be running around 450 to 500 degrees Fahrenheit.

While the grill is heating up, toss your sliced bell peppers and onion rings in the remaining tablespoon of olive oil and season them with a generous pinch of salt and a few cracks of black pepper. Load them into your grill basket or cast iron skillet and set them aside. Getting the vegetables ready now means you can manage everything on the grill at the same time without scrambling.

Place the marinated chicken thighs directly over the hot coals or the direct heat zone of your gas grill. You should hear an immediate and aggressive sizzle when the meat hits those grates. That is exactly what you want. Grill the chicken thighs for 5 to 6 minutes on the first side without touching them. Let the fire do its work and build that deep, gorgeous char. Resist the urge to move the chicken around. Those grill marks develop from sustained contact with the hot grates.

Flip the chicken thighs and grill for another 5 to 6 minutes on the second side. If any pieces look like they are getting too dark on the outside before they are cooked through, slide them to the indirect heat zone to finish. At this point, place your grill basket or cast iron with the peppers and onions directly over the hot zone. Toss them every couple of minutes until they are lightly charred, softened, and smelling incredible, roughly 8 to 10 minutes total.

Check the internal temp of your chicken thighs using your instant read thermometer inserted into the thickest part of the meat, avoiding any contact with the pan if you slid them to indirect. You are looking for a solid 165 degrees Fahrenheit. Do not pull the chicken until you hit that number. Once every piece registers at or above 165 degrees, transfer the chicken to a clean cutting board and let it rest the meat for a full 5 minutes. Do not skip this step. Resting allows the juices to redistribute through the fibers so every slice stays moist.

While the chicken is resting, toss your flour tortillas directly onto the grill grates over medium or indirect heat for about 30 seconds per side. You want them warm, pliable, and showing just a touch of char on the edges. Remove them and wrap in a clean kitchen towel to keep them soft and warm until serving time.

Once the chicken has rested, slice it against the grain into thin strips about a quarter-inch thick. Cutting against the grain shortens the muscle fibers and is the single biggest factor in getting tender, easy-to-eat fajita meat. Arrange the sliced chicken on a large platter alongside the grilled peppers and onions. Squeeze a couple of lime wedges over everything, scatter the fresh cilantro on top, and serve immediately with the warm tortillas and your choice of toppings.