In a medium bowl, break up the tuna with a fork. Add mayonnaise, Dijon mustard, red onion, celery, and lemon juice. Stir until everything’s well combined. Season with salt and pepper.
Lay out the bread slices. Pile the tuna salad onto two slices. If you like a little crunch, add a lettuce leaf on top.
Place the remaining bread slices on top. Slice in half and serve immediately.