Preheat the grill to medium-high; lightly coat the grate with cooking oil. Grill the chicken, turning as needed, for 15 minutes or until cooked through and well-marked. Transfer to a plate. Reduce the grill to medium-low. Spread the mayonnaise just below the center of the tortillas, leaving space on the sides. Top with spinach, tomato, bacon, a cooked chicken breast, Cheddar and avocado.
TIP: Use a Greek-style yogurt based sauce in place of the mayonnaise.