In a large mixing bowl, combine the lime juice, oil, minced garlic, ground cumin, salt, chili powder, and chopped cilantro (if using). Add the chicken thighs and toss until well coated. Cover the bowl and let the chicken marinate for at least 20 minutes—if you have more time, an hour or even overnight in the fridge will deepen the flavors.
While the chicken marinates, wash and cut the bell peppers into thin strips. Peel and thinly slice the red onion. Set these aside so they’re ready when the chicken is cooked.
Heat a large skillet over medium-high heat. Once hot, add the marinated chicken thighs. Cook for about 5-7 minutes per side, or until the chicken is cooked through and has a nice sear. Remove the chicken from the pan and let it rest for a few minutes before slicing it into strips.
In the same skillet, add the bell peppers and red onions. Sauté for 5-6 minutes until they’re tender but still have a slight crunch. Season with a pinch of salt if desired.
Return the sliced chicken to the skillet with the vegetables and toss everything together for a minute or two to marry the flavors. Serve immediately with warm tortillas, sliced avocados, and any other favorite toppings like sour cream, shredded cheese, or salsa.