How to make
Cut the cleaned prunes into halves and remove their pits. In a large tray, pour the sugar over the prunes and let them sit for several hours. Mix the vinegar and water, and sprinkle them with the mixture.
Put the tray in the oven and bake the prunes until the syrup thickens. Bake the marmalade for about 2 hours. Check for the desired density by putting a drop on a porcelain plate. If baked enough, the drop will not run.
Then add the cinnamon, walnuts and citric acid. Stir the marmalade well and while still hot, distribute it into dry and warmed jars. Seal the jars and store them in a cool, dark place.
