For the chicken:
1-1/2 lbs. chicken tenderloins or boneless, skinless chicken breasts
1/4 cup vegetable oil
3 cloves garlic, minced
1 tbsp. lime zest, from 2 limes
1 tsp. cumin
3/4 tsp. oregano
3/4 tsp. ancho chili powder
1/2 tsp. smoked paprika
1 tsp. salt
8-12 small soft flour tortillas
For the peppers:
2 tbsp. vegetable oil
1 large red onion, sliced 1/4″ thick
2 bell peppers, sliced 1/4″ thick
1/4 tsp. salt
1/2 tsp. sugar
Sides for serving (optional):
If using chicken breasts, pound each breast individually to an even 1/2-inch thickness in a sealable bag. (Skip this step if using chicken tenderloins.)
Place chicken in a medium bowl. Add the oil, garlic, lime zest, cumin, oregano, ancho chili powder, smoked paprika and salt into a bag. Blend the marinade together. Add the chicken and seal shut, releasing any air in the bag. Massage the marinade into meat until evenly coated.
Add the chicken back to the bowl, cover, and place in the refrigerator; let the chicken marinate 8-24 hours.
Preheat to high.
While the grill heats, add the oil to a large non-stick skillet over medium-high heat. Add the sliced onions, peppers, salt, and sugar; cook, stirring occasionally, until the vegetables start to brown (8 -10 minutes). Add 1/4 cup of water to the pan and scrape bottom of pan to ensure no sticking. Continue cooking for three to four minutes, until the water is evaporated and the pan starts to brown again; add 1/4 cup more water and cook a few more minutes until the vegetables are crispy. Season to taste with more salt and sugar if necessary. Set aside.
Oil the grates on the grill. Grill the chicken, covered, for two to three minutes per side. Chicken is done when internal temperature reaches 165ºF. Let the chicken cool slightly, then transfer to a cutting board and cut into 1/2-inch strips.
Warm the tortillas by stacking up to six tortillas on a plate; cover with a damp paper towel, then microwave for 30 seconds to one minute. Serve with the chicken and peppers on a platter, along with optional sides.