Prep the mix-ins: Finely chop celery, red onion, and herbs so they blend smoothly into the salad without big chunks.
Make the dressing base: In a medium bowl, whisk Greek yogurt, Dijon, lemon juice, and olive oil (if using) until smooth. Season with a pinch of salt and pepper.
Add the tuna: Drain the tuna well and flake it with a fork.
Add it to the bowl and gently fold to coat every piece.
Stir in the crunch and brine: Mix in celery, onion, herbs, and capers or pickles. Taste and adjust lemon, salt, and pepper.
Rest for flavor: Let the salad sit for 10 minutes, or chill for 30, so the flavors meld and the onion softens.
Serve your way: Spoon onto toast, tuck into a wrap, pile onto greens, or scoop into lettuce cups or avocado halves.