Wash the prunes and remove their pits. Arrange them in a tray and sprinkle with the sugar. Leave them as is for 3 hours (if you're not in a rush, leave them overnight).
Then pour in 1/2 cup water and put the tray in a preheated 374°F (190 °C) oven. Add the finely chopped walnuts. Bake for 40-45 min.
To check to see if it's ready, scoop a little of the jam, pour it into a plate, make an X with the spoon. If it doesn't merge back together, it's ready.
Take it out of the oven and add 1 tsp citric acid. Stir and while still warm, pour it into jars. Then boil the jars for 10 min.