Score the fat of the duck breast with the tip of a knife in diamond shapes without cutting into the meat. Marinate the duck breasts on the meat side in half the balsamic vinegar for about 15 minutes. Meanwhile, in a large saucepan, melt the onion in the butter without browning it. Add the cauliflower, broken into small florets, and pour in the milk. Bring to the boil and simmer uncovered for about 15 minutes, then drain. Blend into a smooth purée with the cream and 1 tbsp. white truffle flavoured olive oil. Add salt and pepper and the slices of truffle (optional). Keep warm. Drain the duck breasts and reserve the marinade. In a hot non-stick frying pan, brown the duck breasts over medium-high heat for 3 to 4 minutes on each side. When they are cooked, discard the cooking fat and add the honey. Caramelise over high heat on both sides and keep warm. Deglaze the cooking juices with the balsamic condiment from the marinade. Over a low heat, reduce the sauce until you have a "glazed syrup". Cut the duck breasts into slices. Season with salt and pepper. Serve them with the cauliflower purée and the truffle balsamic condiment sauce and give them a final turn of the mill before eating.