We always grilled or broiled them with S&P, maybe a dab of butter, and squeezed some lime juice on them when done. Good eats.
OT, but here's my spanish mackerel story. When I was a kid, my parents and one of my sisters were out fishing and got into a school of spanish mackerel. My mom hooked one, and all of a sudden her pole bent WAY down underneath the side of the boat. The next thing we knew, a 25-lb barracuda comes flying in the other side of the boat along with a half mackerel on the end of my mom's line. Dad grabbed the billy club and started pounding on this barracuda thrashing around in the bottom of the boat, my mom is kicking it, and my sister and I are hanging off the stern trying to decide if we'd be better off in the water. Another boat was anchored near us, and the people on it apparently saw the whole thing, because they were screaming "Kill it! Kill it!" The billy club broke, and my dad finished off the barracuda with a beer bottle. The amazing thing was that the barracuda never opened its mouth. It had hit one of the seats in the boat, and when we reupholsterd the seats later on, we found out the fish had hit the seat so hard it had splintered the 1/2" plywood beneath the cushion.