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1
To make the mackerel, prepare the fish by removing any bones with tweezers.
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2
Remove the skin by laying the fillet, skin-side down, on your work surface.
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3
Insert a sharp knife between the skin and flesh.
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4
Grasping the skin with your fingers, gently saw the knife back and forth, keeping it parallel to the cutting board.
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5
Discard the bones and skin.
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6
You should end up with about 3/4 pound trimmed fish.
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7
In a large nonreactive saucepan over high heat, combine the carrot, garlic, onion, vinegar, water, and spices.
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8
Bring to a boil, then lower the heat and simmer for 5 minutes to blend the flavors.
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9
Decrease the heat to maintain a bare simmer, add the fish, and poach until just cooked through, 6 to 8 minutes.
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10
Pour the fish and the liquid into a nonreactive container (glass is best) and allow to cool for 10 minutes.
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11
For best flavor, refrigerate overnight, though the fish can be used as soon as it has cooled.
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12
When ready to use the fish, remove from the liquid and flake into large chunks.
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13
Reserve 2 tablespoons of the pickling liquid for use in the salad and discard the rest.
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14
To make the salad, wash and dry the watercress, then pick off the tenderest leaves and place in a large bowl.
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15
Trim and thinly slice the radishes and add to the bowl, along with the shallot, nuts, and fish.
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16
Season to taste with salt and cracked pepper.
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17
Combine the olive oil and reserved pickling liquid, then add to the bowl and toss the mixture gently so as not to break up the fish.
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18
Divide among 4 plates and serve.
