In a large bowl, add the tuna. Using a fork, flake the tuna well.
Using the fork, gently stir in the chickpeas, celery, cornichons, green apple or cranberries, hard boiled egg and purple onion.
Toss the avocado with 1 TBSP of the lemon juice.
Toss the avocado into the tuna mixture.
Add remaining lemon juice, celery salt and pepper.
Toss gently using the fork so as not to compress the ingredients.
Store covered in the fridge for 30 minutes to chill.
Serve with lemon slice or celery leaves as a garnish.