In a medium bowl, flake the tuna with a fork. Add mayonnaise, lemon juice, celery, and red onion. Stir for about 30 seconds until evenly coated. Season with salt and pepper.

Place a dry skillet over medium heat. Warm each tortilla for about 20 seconds per side, until pliable with a few golden spots.

Lay the warm tortillas on a cutting board. Spoon half the tuna salad down the center of each, leaving an inch on both ends. Add lettuce if using, fold the sides in, and roll tightly. Cut on a diagonal and serve right away.