Prepare the Tuna Salad. Drain the canned tuna well. In a mixing bowl, combine the tuna with mayo or Greek yogurt, Dijon mustard, celery, and onion. Mix until creamy and season with salt and pepper to taste.
Layer the Wrap. Place your tortilla on a clean surface. Layer a handful of lettuce in the center, followed by sliced tomatoes, and then a generous scoop of tuna salad.
Wrap It Up. Fold in the sides of the tortilla, then roll it tightly from the bottom to create a secure wrap. Cut in half diagonally for a clean, café-style presentation.
Serve or Store. Serve immediately or wrap in foil or parchment for a perfect lunchbox or meal prep item.