Preheat the oven to 200ºC/400ºF/Gas 6, boil the potatoes, in their skins, with the sage until tender. Heat a roasting tin or dry frying pan which is suitable for the oven, over a high heat until very hot. Place the Duck Breasts, skin side down, in the pan for 2 minutes until the fat starts to run then season and cook for another 2 minutes transferring to the oven for a further 10 minutes when this time is up. Turn the oven off, open the door and leave the Duck Breasts at the edge to rest for 10 minutes.
Peel and chop the onions very finely and sauté in olive oil, without browning, for 10 minutes, meanwhile remove the sprigs of sage from the potatoes and mash.

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 if necessary. Remove the Duck from the oven and slice thinly to serve.

Serve with creamed leeks.