01 - Combine tuna, mayonnaise, Dijon mustard, red onion, celery, lemon juice, salt, and black pepper in a bowl. Mix thoroughly until evenly incorporated.
02 - Lay tortillas flat. Distribute half the tuna mixture across the center of each tortilla. Top with half the cheddar cheese and tomato slices if desired.
03 - Fold both sides inward toward the center, then roll tightly from bottom to top, securing the filling inside.
04 - Melt butter in a large skillet over medium heat. Place wraps seam-side down and cook for 2 to 3 minutes per side, pressing lightly with a spatula, until golden brown and cheese has melted.
05 - Transfer wraps to a cutting board, slice diagonally in half, and serve immediately while warm.