In a skillet on medium-high heat, sear chicken breast. Reduce heat and add onions and peppers; cook until softened. Add stock, cover and simmer until chicken just begins to pull apart, an internal temperature of 185ºF. Strain contents and reserve liquid. Pull chicken with onions. Mix in both seasoning blends.
Preheat and grease flat grill to 350°F. 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.