Fillet the fish bait-cutter style. Cut away and discard the thin sliver of brown flesh containing the belly bones. Remove the pin bones.
Sprinkle a third of the salt over a large tray. Lay half the mackerel fillets on it, skin side down. Sprinkle over a third of the salt. Add the second lot of mackerel skin side up and sprinkle on the remaining salt.
Leave for 5 minutes and then rinse off the salt quickly but thoroughly.
Combine all the souse ingredients in a large pan with the 300ml of water and 300 ml vinegar. Simmer for 5 minutes.
Lay the rinsed mackerel out on a large roasting tin. Pour over the hot sousing liquid, which should cover, or nearly cover the fillets.
Cover the tin in foil and bake for 30 minutes at 180 C
Remove from oven and allow to cool. They should be allowed to marinade for 24 hours before eating, 6 hours at the very least!
They will keep in the fridge for week.