In a medium bowl, combine the flaked tuna, mayonnaise, Dijon mustard, diced celery, red onion, and lemon juice. Season with a pinch of salt and several grinds of black pepper. Taste and adjust as necessary.

Lay out a tortilla and place a slice of cheddar in the center. Spoon about 1/4 of the tuna mixture over the cheese, spreading it evenly but leaving room at the edges. Add baby spinach or tomato slices if using.

Fold the sides of the tortilla over the filling, then roll tightly into a snug wrap. Press the seam gently to seal. Repeat for remaining wraps.

Heat a large skillet over medium heat and add half the butter. When the butter foams and begins to smell slightly nutty, place two wraps seam-side down in the pan.

Cook for 2 to 3 minutes per side, pressing gently with a spatula until golden and the cheese is melted. Flip carefully to avoid breakage.

Add the remaining butter and cook the last two wraps.

Transfer to a cutting board and let rest for a minute before slicing diagonally. Serve immediately.