There's an optional step in this easy dish which subtly boosts the flavour without adding salt: wrapping the fish for a day or two in kombu ... place the tuna fillets in it and sear them one ...

5 cups (1.2 L) filtered water 1 piece kombu, about 3 x 3 inches (7.5 x 7.5 cm) 3 to 4 cups (20 to 30 g) bonito (dried skipjack tuna) flakes Combine the water and kombu in a medium saucepan. Heat over ...