To make the dressing, whisk together all the ingredients and set aside.
Combine all the marinade ingredients for the chicken and mix well. Pour over the chicken and allow to marinate for 10 minutes.
Heat a griddle pan over high heat and cook the chicken on both sides until cooked through.
Remove and allow to rest for 5 minutes.
Griddle the bread until toasted.
To assemble the sandwich, spread a slice of bread with a generous spoonful of the dressing topped with lettuce, sliced chicken and avocado. Sandwich with another slice of bread and serve.