Begin by trimming the leaves from the beetroot, leaving about 2.5cm of stalk on each one. Place in a large saucepan and cover with water. Bring to the boil, reduce the heat and leave to simmer gently, partially covered, for 40 minutes. Drain and set aside.
Finely grate the zest of one of the lemons into a bowl and squeeze in the juice from both, passing it through a fine sieve, to ensure no pith or pips remain.
Finely chop the dill and onion, and add to the bowl with the sugar, olive oil, capers and 1 teaspoon of sea salt. Whisk all of the ingredients together until fully amalgamated.
Run the beetroots under a cold tap and gently slide off the skins (they should peel off easily). Trim the tops and bottoms from the beetroots and discard. Then, using a mandolin or a sharp knife, slice the beetroot into very thin rounds.
Arrange the beetroot slices on a serving platter or individual plates and spoon over the dressing, letting it sink into every crevice of the beetroots. Cover with clingfilm and chill in the fridge for at least 2 hours before serving.