In a small bowl, whisk together all marinade ingredients until well combined. Reserve 3 tbsp of the marinade for the vegetables.
Coat the Chicken
Pat chicken thighs dry with paper towels and place in a large bowl. Pour the remaining marinade over the chicken and toss to coat evenly.
Cook the Chicken
Heat 2 tbsp oil in a large skillet over medium-medium-high heat. Add chicken thighs in a single layer, avoiding overcrowding (cook in batches if needed). Sear both sides until golden brown and the internal temperature reaches 165°F. Transfer to a plate, and let rest for 5 minutes.
Sauté the Fajita Mix
In a bowl, toss bell peppers and onion with the reserved 3 tbsp of the marinade until coated. In the same skillet over medium heat, sauté vegetables until softened but vibrant, or to your desired texture.
Finish and Serve
Slice rested chicken against the grain into strips. Return chicken to the skillet with vegetables and stir to combine. Season with salt to taste and drizzle with fresh lime juice. Turn off heat, sprinkle with chopped cilantro, and stir again.