The easiest thing to do is to reduce red wine, port and stock with a dollop of whatever fruit or fruit jam you fancy (cherries would be classic - although blackberries would be seasonal). Serve the magret on top of the sauce to preserve the crisp skin.
Alternatively, if you don't fancy fruit, go with some chopped shallots, mushrooms, flambée with brandy and finish with double cream or creme fraiche.