01 - Combine tuna, mayonnaise, Dijon mustard, red onion, celery, lemon juice, salt, and black pepper in a bowl. Mix until thoroughly incorporated and creamy.
02 - Lay tortillas flat. Distribute half the tuna mixture onto the center of each tortilla, spreading evenly. Top each with half the cheddar cheese and tomato slices if desired.
03 - Fold both sides of each tortilla inward toward the center, then roll tightly from bottom to top to form a secure cylinder.
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 is fully melted.
05 - Slice each wrap in half diagonally and serve immediately while warm.