Classic Mayo-Dressed Tuna Salad Sandwiches Recipe

In a small bowl, combine onion and vinegar (vinegar should just cover onion; add more if it does not) and let stand 15 minutes. Meanwhile, in a medium bowl, combine tuna and mayonnaise and mix very well with a spoon until tuna is reduced to very small shreds. Drain onion well, then add to tuna along with celery, parsley, and fish sauce or anchovy. Mix until well incorporated, then season with salt and pepper. Add more mayo, 1 tablespoon at a time, if desired. Spoon tuna salad onto 4 bread slices, top with lettuce leaves, then close sandwiches. Serve right away.