Time 25 minutes

Serving 4

• 1 medium red onion (about 8 ounces; 225g), cut into ¼-inch dice
• ½ cup (120ml) red wine vinegar
• 4 (5-ounce; 140g) cans oil-packed tuna, drained
• 1 cup (240ml) mayonnaise, plus more as needed
• 2 stalks celery (about 6 ounces; 170g total), cut into ¼-inch dice
• 1 ounce (about 10 stalks) flat-leaf parsley leaves and tender stems, minced (about ¼ packed cup)
• ½ teaspoon (3g) Asian fish sauce or very finely minced oil-packed anchovy fillets (about 2 small fillets)
• Kosher salt and freshly ground black pepper
• 8 slices sandwich bread, very lightly toasted
• Romaine lettuce leaves, torn, for garnish

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.

Step 4
Add more mayo, 1 tablespoon at a time, if desired.

Step 5
Spoon tuna salad onto 4 bread slices, top with lettuce leaves, then close sandwiches. Serve right away.