Prep the base: Add drained tuna to a large bowl.
Use a fork to flake it into small pieces so it mixes evenly.
Chop the fresh ingredients: Dice the avocado, cucumber, celery, and onion. Add them to the bowl along with chopped parsley or cilantro.
Mix the dressing: In a small cup, stir together lemon juice, Dijon, olive oil, and garlic. If using, add mayonnaise or Greek yogurt for a creamier texture.
Toss gently: Pour the dressing over the bowl.
Fold everything together with a spatula, keeping the avocado pieces mostly intact.
Season to taste: Add salt and plenty of black pepper. If it needs more brightness, squeeze in extra lemon.
Serve your way: Eat it as-is, spoon it over greens, tuck into lettuce cups, or pile onto toasted sourdough. It’s also great in a whole-grain wrap.