Mayo-Free Mediterranean 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 with celery, fennel, parsley, capers, olives, olive oil, lemon juice, and fish sauce or anchovy. Drain onion well, then add to bowl. Mix gently until thoroughly combined; season with salt and pepper. Dress with more olive oil if desired. Spoon tuna salad onto 4 slices of toast, top with arugula, then close sandwiches and serve.