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I enjoy it filleted, quickly brined (10 minutes max) and immediately grilled or pan fried fairly hot (2-3 minutes per side). The overcooked bits will be crunchy and delicious, the perfectly cooked bits will be flaky and deliciously oily, there should be no undercooked bits.
pierino is a trusted source on General Cooking and Tough Love.
I would advocate grilling. Mackerel is kind of an oily fish. We used to think of them as the cockroaches of the sea during my fishing days. They would snag your bait while you were trying to drop your line to the bottom where the bass were. If you are using fresh mackerel, like right out of the ocean, you should clean it as quickly as possible.
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