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 combined.

Taste and adjust seasoning if necessary.

Lay out 4 slices of bread on a clean surface.

Spread a generous amount of the tuna salad mixture onto each slice of bread.

Top with lettuce leaves and sliced tomatoes if desired, then place the remaining slices of bread on top to complete the sandwiches.

Cut the sandwiches in half and serve immediately.