Prep your mix-ins. Finely chop the basil, celery, red onion, and cucumber. Rinse and chop capers if using. Zest the lemon first, then juice it.

Combine the dressing. In a medium bowl, whisk the Greek yogurt, olive oil, Dijon, lemon zest, and 1 tablespoon of lemon juice. Add a pinch of salt and several grinds of black pepper.

Add the tuna. Flake the drained tuna into the bowl. Use a fork to break it into bite-size pieces without over-mixing. You want a light, flaky texture.

Fold in the veggies and basil. Gently mix in the celery, onion, cucumber, basil, and capers. Taste and adjust with more lemon juice, salt, or pepper. The flavor should be bright and lightly tangy.

Toast the bread. Lightly toast the bread for structure and crunch. This helps prevent sogginess, especially if you’re packing the sandwich for later.

Layer smart. On the bottom slice, add a layer of leafy greens to create a moisture barrier. Spoon on a generous amount of the tuna mixture. Add tomato and avocado slices if you like. Top with the second slice of bread.

Press and cut. Gently press the sandwich to help it hold together. Cut in half diagonally for easy handling.

Pack it right. If taking to work, wrap snugly in parchment or a reusable sandwich wrap. Keep chilled until lunch.