01 - Combine tuna, mayonnaise, Dijon mustard, red onion, celery, lemon juice, salt, and black pepper in a bowl. Mix until thoroughly blended.
02 - Lay tortillas flat. Spread half the tuna mixture onto the center of each tortilla. Sprinkle half the cheddar cheese over each wrap and add tomato slices if desired.
03 - Fold in the sides of each tortilla and roll up tightly to form secure wraps.
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 until golden brown and cheese is melted.
05 - Slice each wrap in half diagonally and serve immediately while warm.