For the chicken and peppers:
3 pounds boneless, skinless chicken breasts
Freshly ground black pepper
1/3 cup plus 2 tablespoons vegetable oil, divided
1/4 cup freshly squeezed lime juice (from about 3 limes)
1 tablespoon chili powder
1 1/2 teaspoons ground cumin
2 cloves garlic, finely chopped
16 to 20 small flour or corn tortillas
3 medium bell peppers, cut into 1/2-inch slices
2 medium yellow or red onions, halved and cut into 1/2-inch slices
Guacamole or sliced avocado
Slice the chicken breasts in half horizontally (also known as butterflying). Season the chicken all over with salt and pepper.
Whisk 1/3 cup of the oil, lime juice, chili powder, cumin, and garlic together in a 9 by 13-inch baking dish. Place the chicken pieces one by one into the marinade, flipping each one to coat. Arrange the chicken into an even layer, it's okay that they stack and overlap. Let sit at room temperature while you heat the oven, 20 to 30 minutes.
Arrange 2 racks to divide the oven into thirds and heat to 500°F. Place a rimmed baking sheet in the oven while it heats.
Carefully take the baking sheet out of the oven. Remove the chicken from the marinade, letting the excess marinade drip off, and place in a single layer on the hot baking sheet.
Place the chicken on the upper rack and roast until just cooked through or it registers 165°F on an instant-read thermometer, about 10 minutes. Meanwhile, place 2 large sheets of aluminum foil on a work surface. Stack 8 to 10 tortillas on each sheet of foil and wrap completely in the foil. Place the bell peppers and onions in a large bowl, drizzle with the remaining 2 tablespoons oil, season with salt and pepper, and toss to coat.
Transfer the cooked chicken to a large plate or cutting board, tent loosely with foil, and set aside to rest. Pour off any excess liquid on the baking sheet. Transfer the bell peppers and onions onto the now-empty baking sheet and spread into an even layer.
Place the baking sheet on the upper rack and place the foil packets of tortillas on the lower rack. Roast until the bell peppers and onions are softened, about 10 minutes.
Slice the chicken into 1/2-inch wide pieces across the grain and transfer to a serving platter. Transfer the bell peppers to a serving bowl or platter. Serve with the warmed tortillas, guacamole or avocado, salsa, sour cream, and lime wedges.