Makes about 4 cups
Time commitment about 1 1/2 hours
5 pounds black or red plums
2 cups water
1 1/3 cups sugar
2/3 cup plus 2 tablespoons apple cider vinegar
6 tablespoons orange marmalade
1 large cinnamon stick
2 star anise
2 whole cloves garlic, peeled and lightly scored with the tip of a knife
4 teaspoons kosher salt
Wash and stem the plums and lay them in a single layer in the bottom of a large Dutch oven. Add the water. Cover and bring to a boil over high heat. Reduce the heat and simmer for 30 minutes, stirring occasionally to prevent the fruit from sticking to the pot. The skins will burst and the plums will release their juice and soften.
Remove the pot from the heat, uncover it, and let the fruit cool for about 5 minutes. Pour the plums into a heavy-duty sieve, a small-holed colander, or a food mill set over a large mixing bowl and push the plums through to render the juice and the pulp and to separate out the skins and the pits. Discard the skins and pits. Return the juice and the pulp to the pot and add the sugar, vinegar, marmalade, and salt. Tie the cinnamon, star anise, and garlic into a piece of cheesecloth and add them to the pot as well.
Set the pot over high heat and bring it just to a boil. Lower the heat and simmer the mixture, uncovered to reduce it, being careful not to let it splatter. Stir often, modifying the heat as needed to keep it at a slow simmer as the liquid reduces. Cook for about 35 minutes, until the mixture has thickened. Draw a spoon across the bottom of the pot; the mixture should be thick enough to part, expose the bottom of the pot, and then come back together again. Note that the catsup will thicken as it cools. Remove and discard the cinnamon, anise, and garlic. Pour the catsup into clean glass jars.
Either process for 10 minutes in a hot water bath to store on the shelf, or refrigerate up to a year.