In a large mixing bowl, mash half the avocados. Add the Greek yogurt, Dijon mustard, garlic (if using), lemon juice, and lemon zest. Whisk together until well combined (or blend in blender or food processor for a smoother consistency).
Add the chicken, celery, green onions, salt, and pepper on top and stir to combine. Cover the mixture and store in the fridge for at least 4 hours to set.
When ready to serve, fold in the remaining chopped avocado and top with fresh cilantro, if using. Enjoy it on its own, as a dip, in a croissant, or on whole grain bread as an avocado chicken salad sandwich.
Notes
Storage: Keep leftover salad in an airtight container in the fridge. It will keep for up to two days. When ready to eat, just assemble the sandwiches.