Preheat the oven to 230 C, Gas 8. Season the mallard well, then smear the breast with the butter. Put into a roasting tin and pour half the stock around it.
Roast in the oven for 35 minutes, basting frequently with its juices. Add more stock if the pan threatens to burn dry. When cooked, transfer it to a warmed serving dish and keep warm.
Pour the roasting juices into a jug, leaving just enough to moisten the base of the tin. Let the juices settle for a couple of minutes and skim off any fat.
Put the roasting tin over a low heat and sprinkle with the flour. Stir until lightly browned, scraping in all the residues.
Add the remaining ingredients to the pan, including the cooking juices. Bring to the boil, stirring contantly, and simmer for a few minutes until thickened.
Taste and check seasoning then serve with the duck.