Step 1

In a small bowl, combine onion and vinegar (vinegar should just cover onion; add more if it does not) and let stand 15 minutes.

Step 2

Meanwhile, in a medium bowl, combine tuna and mayonnaise and mix very well with a spoon until tuna is reduced to very small shreds.

Step 3

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.

Step 4

Spoon tuna salad onto 4 bread slices, top with lettuce leaves, then close sandwiches. Serve right away.