Halve the avocado and remove the pit. Using an ice cream scoop or dessert spoon, carefully scoop out some avocado to double the size of the hole left by removing the pit. Roughly dice the scooped out avocado and place in a medium bowl. Add the chicken, mayonnaise, red celery, onion, lemon juice, lemon zest and chopped dill and combine well so all the ingredients are well distributed.
Place half the chicken salad mixture in the cavity of each avocado half. Garnish each with a sprig of dill.
If assembling ahead of time, leave the avocado whole until a few hours before serving.