In a mixing bowl, combine the drained tuna, mayonnaise, chopped celery, chopped red onion, lemon juice, salt, and black pepper. Mix well using a fork until all ingredients are evenly combined.

Lay out 4 slices of bread. If using, place a lettuce leaf and a slice of tomato on each slice.

Spoon the tuna salad mixture evenly over the lettuce and tomato. Top with the remaining slices of bread.

Cut the sandwiches in half and serve immediately.