In a mixing bowl, combine the fresh squeezed lime juice, minced garlic, ground cumin, salt, and ground chili. Add the skinless and boneless chicken thighs, making sure they are well coated in the marinade. Let the chicken marinate for at least 30 minutes.
While the chicken is marinating, prepare the bell peppers and red onion. Cut the bell peppers into strips and thinly slice the red onion. Set aside.
Heat a skillet or griddle over medium-high heat and add 1 tablespoon of oil. Once hot, add the marinated chicken thighs to the skillet. Cook for about 5-7 minutes on each side until the chicken is cooked through and has a nice sear. Remove from the skillet and let it rest for a few minutes before slicing it into strips.
In the same skillet, add the remaining tablespoon of oil. Add the sliced bell peppers and red onion, and sauté for about 5-7 minutes until they are tender and slightly caramelized. Season with a pinch of salt.
To assemble, place a few strips of chicken on a warm tortilla, top with sautéed veggies, and add slices of avocado. If desired, sprinkle with fresh chopped cilantro for an extra pop of flavor.