Prep the produce. Finely chop the celery, mince the onion, and dice the cucumber.
Chop your herbs. Set aside.
Mash the avocado. In a medium bowl, scoop in the avocado and mash it with a fork until mostly smooth with a few small chunks.
Add the flavor base. Stir in lemon juice, Dijon mustard, olive oil (if using), and a pinch of salt and pepper. Taste it now so you can adjust the acidity and seasoning.
Fold in the tuna. Drain tuna well to avoid a watery salad.
Add it to the bowl and gently break it up, folding it into the avocado mixture.
Add crunch and brightness. Mix in celery, onion, cucumber, and capers (if using). Sprinkle in the herbs. Fold gently to keep some texture.
Season to finish. Taste and add more lemon, salt, or pepper as needed.
Add a pinch of crushed red pepper or paprika for a little heat or color.
Serve your way. Pile onto gluten-free toast or crackers, tuck into lettuce cups, or spoon over greens. Add extra herbs or a drizzle of olive oil on top if you like.