Remove all the pin bones from the mackerel. Spread a third of the mint on the bottom of the dish. Mix the salt, sugar and pepper, and sprinkle a third of it over the mint. Place one layer of fillets on top, skin side down. Sprinkle half the remaining seasoning over the fillets, and lay half the mint on top. Sprinkle on the cognac, if using it.
Place the other fillets on top, skin side up. Sprinkle with the rest of the seasoning, and put the rest of the mint on top. Cover with foil, and weigh down with tins. Refrigerate and keep for up to 3 or 4 days, although the mackerel is good to eat after just 8 hours or so, and can be prepared in the morning to serve at night. To serve it, wipe off the herbs and seasoning, and slice into oblique or vertical slices. A sweet mustard mayonnaise is the traditional accompaniment.
© 2000 Frances Bissell. All rights reserved.