01 - Combine tuna, mayonnaise, Dijon mustard, red onion, celery, lemon juice, salt, and black pepper in a bowl. Mix until thoroughly incorporated.
02 - Layout tortillas on a clean surface. Distribute half the tuna mixture onto the center of each tortilla, spreading evenly.
03 - Sprinkle half the cheddar cheese over each wrap. Place tomato slices on top if desired.
04 - Fold both sides toward the center, then roll up tortillas tightly from bottom to top, securing the filling inside.
05 - 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.
06 - Remove from skillet and slice each wrap in half diagonally. Serve immediately while warm.