In a medium mixing bowl, add the drained tuna, mayonnaise, chopped celery, chopped red onion, and lemon juice. Use a fork or spoon to mix until everything is well combined and the tuna is fully coated.

Season the tuna mixture with kosher salt and freshly ground black pepper to taste. Stir to blend the seasonings throughout.

Lay out the slices of sandwich bread. Spread the tuna salad mixture evenly over two slices. If using, top with lettuce leaves.

Cover each sandwich with the remaining slices of bread. Press down gently and cut each sandwich in half.

Serve immediately, or wrap in parchment paper or plastic wrap for lunch on the go.