In a medium bowl, whisk together the marinade ingredients, the lime zest & juice, 2 tablespoons oil, garlic, soy sauce, brown sugar, 1/2 teaspoon salt, and 1/2 teaspoon pepper, cayenne, and jalapeno. Reserve 1/2 marinade in a small bowl; set aside. Place the chicken and the remaining marinade in Ziploc set aside for 1/2 hour; may marinade longer, but refrigerate. Brush both sides of the onion rounds and the peppers with the remaining 2 tablespoons oil and season with 1/4 teaspoon each of salt and pepper.

Prepare grill and bring a large piece of foil to tent the fajitas with and a clean kitchen towel to wrap tortillas in when they come off the grill.

• Build a two-level fire using coals. Using a large chimney starter, ignite 6 quarts of charcoal briquettes and burn until the coals are fully ignited, about 20 minutes. Empty the coals into the grill, spreading them in a single layer; place an additional 20 unlit coals over the lit coals on one side of grill to create a two-level fire. Position the grill grate over the coals and heat the grate for 5 minutes; scrape clean with a grill brush.
• Build a two-level fire with gas: For a gas grill, light all burners and turn to high, cover, and heat the grill until hot, about 15 minutes; scrape the grill grate clean with a grill brush. Leave one burner on high heat while turning the remaining burner(s) down to medium.

To grill the vegetables:

Place the onion rounds and peppers (skin side down) on the cooler side of the grill. cook the onions until tender and charred on both sides, 10 to 12 minutes, turning every 3 to 4 minutes. Meanwhile, cook the peppers until spottily charred and crisp-tender, 8 to 10 minutes, turning once or twice as needed;

To grill the chicken:

Remove the chicken from the marinade and place it smooth side down on the hotter side of the grill; discard the remaining marinade. Cook the chicken until it’s well browned, 4 to 5 minutes; using tongs, flip the chicken and continue grilling until it’s no longer pink when cut into with a paring knife or an instant-read thermometer inserted into the thickest part registers about 160 degrees, 2 to 3 minutes longer.

As the chicken and vegetables are done, transfer them to a large plate; tent with foil to keep warm.

To grill tortillas:

Working in 2 or 3 batches, place the tortillas in a single layer on the cooler side of the now-empty grill and cook until warm and lightly browned, about 20 seconds per side (do not grill too long or the tortillas will become brittle). As the tortillas are done, wrap them in a kitchen towel or a large sheet of foil.

To serve:

Separate the onions into rings and place them in a medium bowl; slice the bell peppers lengthwise into ¼-inch strips and place them in the bowl with the onions. Add 2 tablespoons reserved unused marinade to the vegetables and toss well to combine. Slice the chicken into ¼-inch strips and toss with the remaining 2 tablespoons reserved marinade in another bowl; arrange the chicken and vegetables on a large platter and serve with the warmed tortillas and desired accompaniements.

Notes:

• On chicken; if the breasts are the super thick monsters so often found these days, may be cut in 1/2 horizontally before pounding.
• On heat level; the heat level is only mildly spicy. Up the heat with additional jalapeno and/or include ribs and seeds