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For the dressing, tip the shallots into a bowl with the vinegar, season and set aside to soften for a few mins. Mix the diced beetroot and olive oil into the shallots. The dressing can be made up to a day ahead and chilled.
To serve, lightly toss the leaves with the mackerel and some of the dressing. Place 3 slices of beetroot onto each plate, pile the salad into the centre, then spoon the rest of the dressing around the outside. Spoon little blobs of horseradish around each plate and serve.