Boil the potatoes in their skins until tender in plenty of salted water with two of the sprigs of sage. Preheat the oven to Gas Mark 6/400F/200C. Heat a dry frying pan suitable for the oven (or a roasting tin) over a high heat until very hot. Place the duck breasts, fat side down, in the oven for two minutes until the fat starts to run. Season the top fat-less sides with salt and pepper. Continue to cook for 2 minutes and then transfer to the oven and roast for a further 10 minutes. Turn the oven off, open the door and leave the duck breasts on the edge to rest for 10 minutes. Peel and chop the onions as finely as possible. Saute in three tablespoons of olive oil for 10 minutes without browning. Meanwhile remove the sprigs of sage, discard and mash the potatoes. Add the potatoes to the onions along with the milk, beating all the time. When you reach a suitably runny consistency, season with salt and pepper and stir in a teaspoon of chopped sage leaves. Add more sage to taste and serve with the duck, cut into slices. I serve this with french beans but you could also use broccoli.