In a mixing bowl, combine the drained tuna, mayonnaise, lemon juice, salt, and black pepper.
Use a fork to mix the ingredients until well combined.
Stir in the chopped celery and onion until evenly distributed.
Lay out the slices of bread on a cutting board.
Spoon the tuna mixture onto two of the bread slices, spreading it evenly.
Top each tuna layer with a leaf of lettuce and a slice of tomato.
Place the remaining slices of bread on top to form sandwiches.
Cut each sandwich in half with a knife and serve immediately on a plate.
Notes
For a more flavorful sandwich, consider adding a pinch of dill or a teaspoon of Dijon mustard to the tuna mixture. Using freshly baked bread can elevate the taste, and for added crunch, you can lightly toast the bread before assembling the sandwich.