In a skillet on medium-high heat, sear chicken breast. Reduce heat and add onions, cook until translucent. Add stock and fajita marinade & seasoning, cover and simmer until chicken just begins to pull apart, with an internal temperature of 185ºF. Strain contents and reserve liquid.
Pull chicken with onions. Mix in crushed chipotle pepper, queso fresco, lime juice, lime zest, and bell peppers. Moisten with some of the reserved liquid, if desired.
Preheat and grease flat grill to 350ºF. Place tortillas on grill. Distribute cheese evenly and top with an even amount of chicken mixture.
When cheese is melted, fold in half and cut in quarters. Serve with lettuce and tomato.