01 - Combine tuna, mayonnaise, Dijon mustard, red onion, celery, lemon juice, salt, and black pepper in a bowl. Mix until thoroughly incorporated.
02 - Lay tortillas flat. Distribute half the tuna mixture evenly onto the center of each tortilla. Top each with half the cheddar cheese and add tomato slices if desired.
03 - Fold both sides inward toward the center, then roll the tortilla 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 2-3 minutes per side, pressing gently with a spatula, until golden brown and cheese has melted completely.
05 - Slice each wrap diagonally in half and serve immediately while warm and crispy.