For the marinade:
1/3 cup coarsely chopped fresh cilantro
1/4 cup lime juice
3 cloves minced garlic
5 tablespoons olive oil
1 teaspoon ground cumin
1 teaspoon dark brown sugar
1/2 teaspoon kosher salt
3 boneless, skinless chicken breasts, pounded to 1/2-in thick
For the vegetables:
1 large red bell pepper, cut into 1/4-inch strips
1 large green bell pepper, cut into 1/4-inch strips
1 large white onion, cut into 1/4-inch strips
1. In a small bowl, mix together all of the ingredients for the marinade. Place the chicken breasts in a large Ziplock bag and pour marinade all over. Seal the bag, removing as much air as possible and allow to marinate for 30 minutes in the refrigerator.
2. Light one chimney full of charcoal. When all the charcoal is lit and covered with gray ash, pour out and spread the coals evenly over the charcoal grate. Remove the chicken from the marinade and cook over high heat until browned on both sides and cooked through, about three to five minutes per side. Remove the from the grill and allow to rest five to ten minutes. Slice into 1/2 inch strips.
3. While the chicken is resting, toss the peppers and onion with a little olive oil and a pinch of salt. Place a cast iron skillet directly on the hot coals in the grill, and when heated, add in the peppers and onions. Continue to cook, stirring frequently, until they are soft and nicely browned, about 5 to 10 minutes.
4. Heat the tortillas on the grill or in the oven until soft. Assemble fajitas with the vegetables, chicken, and a dollop of sour cream.