In a medium bowl, use a fork to break up the drained tuna, then add the mayonnaise, celery, red onion, lemon juice, mustard, salt, and pepper. Fold together until evenly combined, about 2 minutes.

Toast the bread slices in a toaster, under a broiler, or in a skillet until golden and crisp, 2 to 3 minutes total.

Spoon half the tuna salad onto two slices of toast, spread it to the edges, top with lettuce and tomato, close with the remaining toast, and serve right away.