Spanish Mackeral is about as good as it can get when fresh.
Marinate fillets is a mixture of 80% soy sauce, 5% mirin, 5% plum vinegar, and 10% dry white wine. Add to this mix fresh ground ginger, crushed garlic, and about 2 tbs of brown sugar, I put the sugar in a little bit of hot water to disolve first and lots of fresh green onion sliced lengthwise. Marinate for about a 1/2 hour. Then steam the fillets, if you don't have a steamer I used to use a wire rack in a large aluminum pan. Fill the pan with water so that is it about 1/3 the height of the rack and set the fillets and the onion from the marinade and more green onion on top of the fish. Cover tightly with aluminum foil and once steaming, steam for 6 minutes. You will not be dissapointed. the first time I had fish like this Crazy Al made porgies for me. I got home and tried it on the Spanish and served it to many guests who all said it was amongst the best fish they had ever eaten. I now do seabass and weakfish the same way. Good eats!