Prep your mix-ins. Finely chop the celery, red onion, and herbs. Smaller pieces blend better and give even flavor.
Mash the avocado. In a medium bowl, scoop out the avocado and mash it with a fork until mostly smooth with a few small chunks.
Add flavor boosters. Stir in the lemon juice, Dijon mustard, and olive oil (if using).
Season with a pinch of salt and pepper.
Fold in the tuna. Add the drained tuna, breaking it up gently with a fork. Fold until coated but not overmixed to keep a pleasant texture.
Stir in crunch and herbs. Mix in the celery, red onion, and parsley or cilantro. Add any optional mix-ins like capers or a pinch of garlic powder.
Taste and adjust. Add more lemon, salt, or pepper as needed.
If it’s too thick, a splash of olive oil or an extra squeeze of lemon loosens it up.
Serve your way. Spoon onto toast, tuck into lettuce wraps, top a salad bowl, or serve with cucumber rounds and crackers.