The phrase san-mai ni orosu is often used in association with preparing fish. It means to cut the fish into three pieces: two fillets of flesh and one piece with the backbone.
Before filleting, be sure to clean the fish well. First, remove the scales, then slit right through to the inside of the belly and remove the guts. Use plenty of running water to wash out the blood, any remaining scales and small bones. Drain off the water quickly, then wipe well with a dry cloth.

1 After gutting the fish, use a brush to clean out any remaining slime, etc.

2 Slit the underside by drawing the knife toward you.

3 Insert the tip of the blade first, and cut along the backbone joints to remove the backbone.

4 Use the tip of the blade to carefully cut down as far as the tail.

5 Finally, cut right through to the top of the fish.

Even in the old days before electric refrigeration, there were ways to slow down the deterioration of raw fish. Here are two ways.

1. Beta-jio: Place fillets in a container that contains plenty of salt. Make sure the fillets are completely surrounded by salt. Leave in a cool place (about 10°C). After the salt has absorbed the moisture from the fish, wash it off. Su-arai: Next, soak in vinegar. You can improve the fragrance by adding slices of a citrus fruit to the vinegar.

For this example our chef used a mackerel. This and other fish with shiny bluish backs may cause a bad reaction when eaten raw, but you can prevent this by reducing the bacteria through long periods of salting and soaking in vinegar.

2. Kobu-jime: Measure out salt weighing about 2% of the filleted fish weight, and sprinkle it over the fillets. Leave for 5 to 6 hours. Moisten kombu seaweed with vinegar or saké then wrap it around the fillets.