Cook the unpeeled beetroot in boiling water until tender- about an hour. Drain, reserving 200 ml of the water. Rinse the beetroot under cold running water and rub off the skin and stems. Cut into 1 cm dice.
Return the beetroot to the pan and add the reserved liquid, vinegar, sugar, salt and pepper. Bring to the boil stirring, and allow to bubble until the liquid is reduced to a slightly sticky syrup. Keep warm.
Meanwhile wash and dry the duck breasts and score the skin in a criss cross pattern with a sharp knife. Rub with the salt and thyme leaves.
Place on a heatproof plate that will fit into your steamer and steam over simmering water for 20 minutes, Remove the duck breasts and pat dry. You can do this a few hours ahead if you like.
Heat the oil in a non stick frying pan and sear the duck breasts, skin side down, until the skin is crisp and golden. Turn them over, reduce the heat and cook gently until heated through.
Carve each breast thickly on the diagonal and serve with the beetroot and a leafy green salad.