Using a metal speed peeler, shave the beetroot into a bowl. Add the lemon juice, a little salt and pepper, a good lug of olive oil and the balsamic vinegar, and mix.
On a large plate, wave the salmon around freestyle. Throw on some sprigs of watercress, then the beetroot. Using a sharp grater, such as a microplane, finely grate the horseradish over. Add a little of the beetroot juice and a bit of pepper, then finish with a drizzle of olive oil.