Marinate the chicken: To a large freezer bag, add the chicken strips, chipotle chiles with adobo sauce, olive oil, lime juice, brown sugar, chili powder, cumin, smoked paprika, black pepper, and salt. Mix well so the chicken is coated evenly in the spice mixture. Seal bag and marinate for up to 24 hours.
When ready to cook, heat a large cast iron skillet over medium-high heat until it's hot. Remove the marinated chicken from the marinade (save the marinade for step 4), and place the chicken in an even layer in the skillet. Cook for 5 minutes until chicken begins to brown, then flip the chicken and cook it on the other side for 4-5 minutes. Chicken is done when it reaches an internal temperature of 165 degrees Fahrenheit. Once chicken has finished cooking, remove from skillet, and set aside in a bowl.
To the skillet, add the chopped onions, bell peppers, and the marinade. Cook for 7-8 minutes over medium high heat, until the vegetables begin to soften and brown.
Add the chicken back into the pan with the vegetables, and cook for 4-5 more minutes until chicken and vegetables are hot.
To make your fajitas, warm tortillas in the oven. Add 1/3 cup of chicken and vegetables to each tortilla. And add your favorite toppings: I used red onions, cilantro, avocado, and pico de gallo salsa.
Store leftovers fajita chicken and vegetables in an airtight container and refrigerate for up to 2 days.