Sear the chicken on both sides on a hot grill. Move to an area of the grill that has medium heat to continue cooking.
Place onions and pepper halves on the grill and close the cover. Cook the vegetables until they are browned and tender, turning once.
Cook the chicken for approximately 10 minutes per side (for boneless thighs – breasts may cook more quickly) until the temperature has reached 160°F.
Allow to rest for several minutes after removing the chicken from the grill and to allow the temperature to rise another 5 degrees.
Slice the chicken, onions, and peppers into strips. Place on flour tortillas and top with cheese, lettuce, tomatoes, salsa, sour cream and guacamole.