01 - In a medium mixing bowl, blend the drained tuna with mayonnaise, Dijon mustard, diced celery, diced red onion, lemon juice, and season with salt and black pepper. Stir until evenly combined.
02 - Lay out the flour tortillas on a clean cutting board. With a knife, create a single cut from the center of each tortilla to the edge, forming a foldable quadrant.
03 - Spoon half of the prepared tuna mixture onto one quadrant of each tortilla. Sprinkle a quarter cup of shredded cheddar cheese onto the next quadrant. Arrange tomato slices and avocado (if using) on the third quadrant, keeping the last quadrant empty.
04 - Starting with the tuna quadrant, fold over onto the cheese quadrant, then over the tomato and avocado quadrant, finishing with the empty quadrant to create a layered triangle.
05 - Heat a nonstick skillet over medium heat and melt half the butter. Place one wrap seam-side down in the skillet and cook for 2 to 3 minutes or until golden brown and crispy. Flip and cook the opposite side for an additional 2 to 3 minutes. Repeat with the remaining wrap.
06 - Slice the wraps in half and serve immediately while warm.