In a large shallow bowl, whisk together lime juice, oil, minced garlic, ground cumin, salt, ground chili and chopped cilantro (if using).
Add the chicken thighs to the marinade, turning to coat evenly; if time allows, cover and refrigerate to marinate for 30 minutes.
Heat a 12-inch (30 cm) skillet over medium-high heat until very hot; add a drizzle of oil to lightly coat the pan.
Sear the chicken thighs, flipping occasionally, until well browned and cooked through, about 8 minutes per side depending on thickness; transfer to a plate and loosely tent with foil to rest.
Add the bell pepper strips and sliced red onion to the hot skillet (add a little more oil if needed) and cook, stirring occasionally, until the onion is soft and the peppers are slightly charred and tender, about 5–7 minutes; season with salt and pepper to taste.
Slice the rested chicken into strips, then return to the pan with the peppers and onions briefly to heat through if desired.
Serve the chicken and vegetables with warmed tortillas, sliced avocado and extra chopped cilantro if desired.