Plum Ketchup

4 pounds ripe red plums, pitted
2 medium onions, finely chopped
1 cup white sugar
3/4 cup dark brown sugar
1 1/2 teaspoons salt
11/2 teaspoons cinnamon
1 1/2 teaspoons powdered mustard
3/4 teaspoon grated nutmeg
1/2 teaspoon ground cloves
1/4 teaspoon ground allspice
1 3/4 cups cider vinegar

Place the plums and onions in a nonreactive pot. Cover with a half inch of water. Bring the water to a boil, lower the heat and simmer 20 minutes, or until the ingredients are tender. Drain.

Return the cooked plums and onions to the pan. Add the remaining ingredients. Bring the mixture to a simmer and cook 1 1/2-2 hours, stirring occasionally, until the mixture is very thick. Pour the ketchup into sterilized jars and keep refrigerated or seal in jars prepared according to manufacturer's instructions.

Yield: About 1 quart.