Place the plum slices in a heavy nonreactive saucepan with the currant jelly, brown sugar, orange zest and cinnamon stick. Bring the mixture to a boil, stirring occasionally. Boil gently over low heat, mashing some of the slices against the side of the pan as they cook, until the plums are very tender, about 15 minutes. Remove the pan from the heat. Allow to cool slightl