How to Make It
Puree the plums in a food processor. In a large, heavy pot, mix the plum puree with the sugar and bring to a boil over moderate heat, skimming as necessary. Cook uncovered over low heat for 40 minutes, stirring occasionally.
Stir the orange and lemon juices and the orange zest into the plums; simmer until thickened, about 10 minutes longer. Pour the plum puree into 5 hot sterilized 1-pint jars, stopping 1/4 inch from the top. Wipe the glass rims and close the jars. Set them in a water bath and bring to a boil. Process for 20 minutes.