In a medium mixing bowl, combine the drained tuna, mayonnaise, and Dijon mustard. Add finely diced celery and red onion. Stir until well combined. Season with salt and pepper to taste.

Lay a whole wheat wrap flat. Spoon a portion of the tuna mixture in the center, spreading it out slightly while leaving space around the edges. Top with shredded cheese and lettuce or spinach.

Fold the sides of the wrap inward, then roll tightly from the bottom to encase the filling completely.

Preheat a skillet over medium heat for 2-3 minutes. Place the wrap seam-side down in the hot skillet, cooking for 2-3 minutes until golden. Flip and cook the other side for another 2-3 minutes.

Once both sides are golden and crispy, remove the wrap from the skillet, allow to cool slightly, and slice in half for serving.