All ingredients ready? Let's begin!
In a small mixing bowl, whisk together the olive oil, lime juice, garlic, chili powder, salt, and pepper. Place the chicken breasts in a glass baking dish or 1-gallon plastic baggie. Pour 1/2 of the marinade over the chicken and turn to coat evenly. Reserve the rest for the vegetables. Seal the dish or baggie and refrigerate for 1-1/2 hours and up to 4 hours.
Remove the chicken from the refrigerator 30 minutes prior to grilling and set it aside to come to room temperature.
Lay out your onions and peppers on a sheet pan. Pour the remaining marinade over the onions and peppers, trying not to separate the onion slices into rings. Turn over to coat evenly.
Start your charcoal grill, preheat your gas grill to medium-high, or heat up your stovetop grill pan to medium-high.
Remove the chicken breasts from the marinade, allowing the excess to drip off. Grill the chicken until the internal temperature reaches 165°F when checked with a meat thermometer, approximately 7 minutes on each side. Transfer to a clean cutting board and tent loosely with foil.
While the chicken rests, grill the onions and peppers until tender and slightly charred, approximately 6 minutes per side. The onions might take longer than the peppers. Transfer the onions and peppers to the cutting board.
Separate the rings of the onion slices and place them on a large serving platter. Slice the pepper halves lengthwise into thin strips and place those on the platter. Slice the chicken crosswise into thin strips and place those on the platter.
Quickly warm the tortillas on the grill, a few seconds per side. Place them in a clean kitchen towel to keep warm.
Serve immediately with your preferred toppings