Cook onions in a stainless pan to release juices and simmer, do not brown, oil is not necessary. Add garlic and cook for 2 minutes, add vinegar, turn up heat and reduce to half. Then add tomatoes and simmer for 10 minutes, puree and adjust seasonings.
Dredge mackerel fillets in flour and cornmeal. Saute over high heat in just enough oil not to burn, 1 1/2 minutes per side. Remove fish and wipe out pan. Return fish and add sauce. Cook in pan for 2 minutes more.
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Serve with citrus corn relish and steamed potatoes or pasta.
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Optional: Add 1 tablespoon of lime juicemixed with 1 teaspoon of hot pepper paste to the sauce.
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