For Mayo: In a small bowl combine mayo, cilantro, garlic and 1/2 of the lime juice (more or less to taste) and salt and pepper. Cover and refrigerate while you prepare the rest of the meal.
For chicken: Butterfly chicken breast by slicing throught the middle horizontally to cut almost in half. Open out flat. Brush lightly with oil, sprinkle with a pinch of taco seasoning on each side and salt and pepper to taste.
For outdoor grilling: Grill over medium hot coals for about 3 minutes per side or until chicken is opaque with no trace of pink.
For indoor grilling: Preheat broiler, broil 3 minutes per side or until opaque or preheat a ridged grill pan to medium heat and cook 3 minutes per side until opaque.
Just before the chicken is done squirt with remaining lime juice.
Lightly toast bread on the grill about 30 seconds to 1 minute. Assemble your sandwiches topped with avocado, lettuce, onion, tomato and of course the cilantro lime mayo. Enjoy!