Make the dressing. In a large bowl, whisk together lemon zest, lemon juice, olive oil, Dijon, garlic, oregano, a pinch of salt, and a few grinds of pepper.
Taste and adjust the acidity or salt as needed.
Add the beans. Stir in the drained, rinsed white beans. Let them sit in the dressing for a minute to soak up the flavor.
Fold in the tuna. Add the drained tuna and gently break it into chunks with a fork. Keep some texture rather than shredding it too finely.
Add crunch and color. Mix in the onion, cucumber, and tomatoes.
If using olives or capers, add them now.
Herbs last. Fold in chopped parsley or basil. Herbs brighten the whole dish, so don’t skip them.
Taste and finish. Adjust salt, pepper, and lemon. If it needs richness, add a splash more olive oil.
If it needs pop, another squeeze of lemon does the trick.
Serve. Enjoy it as is, pile it onto toasted sourdough, spoon over greens, or tuck into lettuce cups. A sprinkle of feta or arugula on top is a nice touch.