Mix the tomatoes, red onion and avocado to make a salad. Mix the juice and zest of the lime with the mayonnaise.
In a large bowl, combine the Old El Paso® Spice Mix for Tacos with the oil, add the chicken and turn in the mixture to coat well. Cook chicken breasts on a moderately hot BBQ, griddle pan or grill for 12-15 minutes turning a couple of times. Ensure chicken is fully cooked through then remove and slice into thin strips. Warm the Taco Shells by following the instructions on the box.
Fill the crunchy Taco Shells with chicken, avocado salad, shredded lettuce and a generous dollop of zesty lime mayo and enjoy the crunch!