Bring all the ingredients, except the mackerel, to the boil in a pan then simmer for 30 minutes.
Place the fish, skin side up, in a shallow pan.
Pour over the hot court-bouillon and bring back to the boil. When it has bubbled over, remove it from the heat and leave to cool before chilling overnight. Remove the fillets from the court-bouillon and serve on individual plates and garnish with the sliced carrot and onions, and a few spoonfuls of sieved court-bouillon.
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