01 - Combine drained tuna, mayonnaise, Dijon mustard, red onion, celery, lemon juice, salt, and black pepper in a mixing bowl. Mix until thoroughly incorporated and creamy.
02 - Lay tortillas flat on a clean surface. Distribute half the tuna mixture onto the center of each tortilla, spreading evenly. Top each with half the shredded cheddar cheese and tomato slices if desired.
03 - Fold both sides of each tortilla inward toward the center. Roll from the bottom upward, tucking ingredients tightly to form a secure cylinder shape.
04 - Melt butter in a large skillet over medium heat. Place wraps seam-side down in the pan. Cook for 2–3 minutes per side, pressing gently with a spatula, until golden brown and cheese is thoroughly melted.
05 - Transfer wraps to a cutting board. Slice each in half diagonally and serve immediately while warm and crispy.