Split the 2 sub rolls lengthwise, leaving a slight hinge if you prefer. Lightly toast the cut sides of the rolls in a toaster oven or on a hot skillet for 2–3 minutes until just golden (optional).

Spread about 2 tablespoons (per sandwich) of mayonnaise evenly on the bottom half of each roll and 1/2 teaspoon mustard on the top half; set aside.

Tuna salad

Prepare tuna

Open and drain the canned tuna thoroughly. Place the tuna in a medium bowl and flake with a fork until mostly broken up.

Add 1/4 cup mayonnaise, the diced celery (1/2 cup), 1/4 cup finely diced red onion, and 1 tablespoon pickle relish to the bowl. Mix gently to combine.

Season the tuna mixture with 1 teaspoon lemon juice, 1/2 teaspoon salt, and 1/4 teaspoon freshly ground black pepper. Taste and adjust seasoning if needed. If mixture seems dry, add up to 1 tablespoon more mayonnaise.

Keep the prepared tuna salad chilled until assembly (or at room temperature for up to 30 minutes if serving immediately).

Toppings

Prepare vegetables: thinly slice the tomato and toss the slices lightly with 1/8 teaspoon salt, 1/8 teaspoon black pepper, 1 teaspoon olive oil and 1/2 teaspoon vinegar. Let sit 1–2 minutes to macerate.

Tear or shred about 1 cup lettuce and thinly slice 1/4 cup red onion for topping.

Layering and assembly: On each bottom roll spread, divide the tuna salad evenly between the two rolls (about half the tuna mixture per sandwich). Smooth it into an even layer about 1/2–3/4 inch thick.

Place 2 provolone slices on top of the tuna on each sandwich. Add a layer of tomato slices over the cheese, followed by the shredded lettuce and the thin red onion slices.

If desired, drizzle a few drops of olive oil and a small splash of vinegar over the lettuce and tomatoes for extra brightness.

Close the sandwiches, press gently to compact, and cut each sandwich in half on the diagonal. Serve immediately.