Mix Tuna Salad: In a bowl, combine the flaked tuna, mayonnaise, Dijon mustard, celery, red onion, and lemon juice. Season with salt and pepper to taste.
Prepare Bread: Lightly toast the bread slices, if desired.
Assemble the Sandwich: Spread the tuna mixture evenly over one slice of bread. Top with lettuce or tomato, if using.
Finish and Serve: Place the second slice of bread on top. Slice the sandwich in half and serve immediately.
