In a large mixing bowl, mash half the avocados. Add the Greek yogurt, Dijon mustard, garlic, lemon juice and lemon zest. Whisk together until well combined (or blend in blender for smoother consistency).
Add the chicken, celery, green onions, salt and pepper on top and stir to combine. Fold in the remaining chopped avocado.
Serve with fresh cilantro on its own, as a dip, in a croissant or with whole grain bread.
Notes
Storage: Store any leftovers in an airtight container. They will last up to 2 days.