In a small bowl, stir together the brown sugar, paprika, chili powder, ground cumin and kosher salt. Set aside.
To a large ziploc bag, combine the chicken, olive oil, juice from half a lime, and the spice blend. Seal completely and rub the ingredients into the chicken. Place in the refrigerator to marinate for 1 hour.
In a large nonstick skillet set over medium heat, add a drizzle of oil. Add the mushrooms and cook for about 5-6 minutes or until tender. Add the corn kernels, red bell pepper, butter, and cumin. Cook for 5 minutes. Stir in the green onions and remaining juice from half a lime. Remove from heat and set aside.
In another nonstick skillet set over medium heat, add another drizzle of oil. Remove the chicken from the refrigerator and slice into strips. Place the chicken in the preheated pan. Cook for about 8-10 minutes or until the chicken is completely cooked through. Stir in the veggie mixture and toss to combine.
Serve your fajitas on warmed flour tortillas topped with sour cream, cotija cheese, and cilantro. Enjoy!