2  Valencia oranges
1 1/2 pounds rhubarb, cut into 1-inch lengths
1  large onion, chopped
1/2 cup golden raisins
1/2 cup dark raisins
1 1/2 cups malt vinegar
1 1/2 cups packed brown sugar
1/2 teaspoon ground mace
1/2 teaspoon ground cinnamon
1/2 teaspoon ground allspice

Cut off the top and bottom of each orange down to the flesh. Stand each orange on end and cut off the peel down to the flesh. Seed the oranges and cut them into small pieces, reserving the juice.

In a large nonreactive saucepan, combine the oranges, juice, and all the remaining ingredients. Bring to a boil, then reduce heat to a simmer and cook for 2 to 2 1/2 hours, or until the mixture is thick and dark. Stir frequently to prevent sticking.

Let cool to room temperature. Cover and refrigerate for up to 2 weeks. Serve at room temperature.