Instructions: Put the mackerel fillets in a shallow dish. Pound the garlic, parsley and lemon zest together in a mortar (or use a strudy bowl and the end of the rolling pin), gradually working in the olive oil, lemon juice and seasoning with salt and pepper. Smear this mixture over the mackerel fillets, then leave to marinate for at least an hour.

To make the relish: Slice or chop the rhubarb roughly and mix with all the remaining ingredients. Preheat the grill thoroughly and line the grill rack with silver foil, so that the fillets dont collapse through the bars. Lay the mackerel, skin side up, on the foil and grill close to the heat, until the skin is browned (about 20 minutes), by which time the mackerel should be crooked through. Serve with the relish.

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