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.