In a bowl, combine the drained tuna, mayonnaise, Dijon mustard, lemon juice, salt, and black pepper.
Mix the ingredients with a fork until well combined.
Lay out the bread slices on a cutting board.
Evenly spread the tuna mixture on two slices of bread.
Top the tuna mixture with tomato slices and lettuce leaves.
Add cheese slices if desired.
Place the remaining bread slices on top to complete the sandwiches.
Optionally, toast the sandwiches in a toaster for a warm version.
Cut the sandwiches in half and serve immediately.
Notes
For an added crunch, consider adding chopped celery or pickles to the tuna mixture. If you prefer a lighter option, replace the mayonnaise with Greek yogurt. For a more flavorful sandwich, try using whole grain or sourdough bread.