In a medium bowl, combine the drained tuna, mayonnaise, Dijon mustard, celery, red onion, and lemon juice. Mix until well combined.
Season the mixture with salt and pepper to taste. Adjust the ingredients according to your preference.
Toast the bread slices if desired.
Spread a generous amount of the tuna salad mixture onto two slices of bread.
Top with lettuce leaves and tomato slices if using, then place the other two slices of bread on top to form sandwiches.
Cut the sandwiches in half and serve immediately.