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Lay the mackerel fillets, skinside down, in a shallow dish, then sprinkle with the fennel seeds and 1 1/2 tbsp flaky salt, making sure you cover the fish evenly. Cover with cling film and put in the fridge for 1 hr, then remove, wash off the salt and fennel seeds, and pat the fish dry with kitchen paper.
Return the fish to the shallow dish, skin-side up, pour over the olive oil and scatter over the shallots. Put the sugar, lemon juice and vinegar in a small saucepan and bring to the boil, then pour the hot liquid over the fish. Leave to cool, cover in cling film and put in the fridge to marinate overnight.
The next day, remove the fish from the fridge 1 hr before serving to bring it to room temperature, then transfer to a serving plate. Stir in the olives, celery and celery leaves. Leave for a few mins, then serve with lemon wedges and a couple of spoonfuls of crème fraîche.