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1
Lay the mackerel in a single layer in an 11- or 12-inch skillet, preferably stainless steel.
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2
The pan should be just big enough so that the fish fillets, nestled side by side, cover the bottom.
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3
Pour 2 cups or more water and the lemon juice over the fillets to cover them, and sprinkle half the peppercorns and 1 teaspoon of the salt over.
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4
Set the pan over medium heat, and bring to a slow rolling boil.
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5
Turn down the heat, and simmer gently for 15 minutes.
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6
Prop up a large wooden or plastic cutting board at a moderate angle, tipping toward the sink.
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7
When the fillets are done (to check: the point of a knife should slip through the center easily), remove them carefully from the pan with a slotted spatula and lay them on the board.
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8
Let them drain and cool completely.
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9
Select a shallow glass or ceramic dish that will just hold the fillets in one layer, or you can use a smaller, slightly deeper dish that will accommodate two layers of fillets.
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10
Arrange the mackerel in whichever dish, sprinkle each layer lightly with salt and the remaining peppercorns, and pour in enough of the olive oil to cover the fillets.
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11
If you are using the mackerel the same day, let it marinate for 2 to 3 hours at room temperature.
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12
Otherwise, cover securely with plastic wrap and store for up to a month in the refrigerator.
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13
Bring back to room temperature 2 hours before serving.
