01 - Combine drained tuna, mayonnaise, Dijon mustard, red onion, celery, lemon juice, salt, and black pepper in a bowl. Mix thoroughly until all ingredients are evenly distributed.
02 - Lay tortillas flat on a clean surface. Spread half the tuna mixture over the center of each tortilla, leaving a border around the edges.
03 - Sprinkle half the cheddar cheese evenly over each wrap. Add tomato slices if desired.
04 - Fold in both sides of the tortilla, then roll tightly from bottom to top to create a secure cylinder shape.
05 - Melt butter in a large skillet over medium heat. Place wraps seam-side down and cook for 2 to 3 minutes per side, pressing gently with a spatula, until golden brown and cheese is melted.
06 - Transfer wraps to a cutting board and slice diagonally in half. Serve immediately while warm.