Prepare the Tuna. Open the cans of tuna and drain them very well. Add the tuna to a medium bowl and gently flake it with a fork.
Mix the Tuna Salad. Add mayonnaise, Dijon mustard, lemon juice, salt, and black pepper to the bowl. Stir until everything is evenly combined and creamy. Taste and adjust seasoning if needed.
Prep the Bread. Lay out your bread slices. You can use them soft as-is or lightly toast them for extra structure and texture.
Assemble the Sandwich. Spoon the tuna salad onto one slice of bread. Top with lettuce and tomato if desired, then place the second slice of bread on top.
Serve and Enjoy. Cut the sandwich in half and serve immediately with chips, fruit, soup, or a crisp salad.