01 - In a mixing bowl, combine the drained tuna, mayonnaise, Dijon mustard, red onion, celery, lemon juice, salt, and black pepper. Mix thoroughly until all ingredients are evenly distributed and the mixture is creamy and well blended.
02 - Lay the flour tortillas flat on a clean work surface. Distribute half of the tuna salad mixture onto the center of each tortilla, spreading it evenly. Top each with half of the shredded cheddar cheese. Add tomato slices if desired.
03 - Fold both sides of each tortilla inward toward the center, then roll from the bottom upward to create a tight, secure cylinder. Ensure the seam is facing down to prevent unrolling during cooking.
04 - Melt the butter in a large skillet over medium heat. Place the wraps seam-side down in the pan. Cook for 2 to 3 minutes on the first side, pressing gently with a spatula, until golden brown and crispy. Flip carefully and cook for another 2 to 3 minutes until the cheese is completely melted and the tortilla is evenly toasted.
05 - Transfer the wraps to a cutting board and slice each in half diagonally. Serve immediately while hot and cheesy.