Step 1

Place the orange juice or water, lemon juice, prunes, raisins and sugar in a small saucepan. Cook over low heat, tossing and stirring the mixture to soften and plump the prunes and raisins, for about 8 to 10 minutes. Be sure it doesn't stick to the bottom of the pan; lower the heat if the mixture starts to boil. Remove the pan from the heat and, with a slotted spoon, transfer the fruit to the bowl of a food processor. Reserve the cooking liquid. Let the fruit cool for about 5 minutes.

Step 2

Add the cinnamon and walnuts (if using). Process well to achieve a thick pastelike puree, adding the cooking liquid a bit at a time as needed if the mixture is too thick; too much liquid will make the filling runny. Taste the filling and add sugar (a tablespoon at a time) if required. The filling should be thick and moist.

Step 3

Chill slightly before using. The chilled mixture can be used immediately, or it can be transferred to an airtight container and refrigerated for up to two weeks or frozen for up to six months.