Prepare the filling: In a medium bowl, combine the drained tuna, sweetcorn, mayonnaise, and Dijon mustard if using. Mix gently but thoroughly, breaking up any large chunks of tuna, until all ingredients are evenly coated. Taste and season with a pinch of salt and black pepper to balance the flavors.

Assemble the wraps: Lay each tortilla flat on a clean surface. Place an even layer of shredded lettuce in the center of each tortilla. Add a few slices of tomato and red onion, if using, over the lettuce. Spoon the tuna and sweetcorn mixture on top, spreading it evenly without overfilling to make wrapping easier.

Roll the tortilla: Fold the sides of the tortilla over the filling, then roll it tightly from the bottom up to form a secure wrap. Press gently to hold everything together. You can cut the wrap in half diagonally for easier handling and a nicer presentation.

Serve and garnish: Arrange the wraps on a plate and, if desired, sprinkle with chopped parsley or cilantro for a fresh touch. Serve immediately as a quick lunch, snack, or picnic option. These wraps are best enjoyed fresh but can be wrapped in foil and kept in the fridge for up to a few hours if needed.