Halve and pit the well-washed prunes. Arrange them in a deep tray, sprinkle with the sugar and leave for about 2 hours to release their aromatic juice.
Put them in a preheated 482°F (250 °C) oven until boiling, then lower the oven to 302°F (150 °C). Keep baking until the sugar fully dissolves and there's plenty of juice released. Add the cinnamon sticks and coarsely chopped walnuts.
After about 1 hour 30 min. of baking in a moderate oven, turn it off and leave the jam to cool gradually.
Put the cooled mixture on the stove and heat it gradually. Boil on moderate heat to the desired consistency, stirring very carefully so the fruits don't turn to mush.
Once you're satisfied with the consistency, add the citric acid, boil another 5-6 min. and distribute the hot jam into dry jars. Close the jars and put away in a dry and cool area.