Place prunes in a saucepan, cover with water and bring to a brisk bubble. Turn heat down and simmer prunes gently, uncovered, until soft, about 10 minutes.
Drain the prunes, reserving 1 tbs of liquid and place the fruit, lemon juice, liquid, and sugar in a food processor. Process until smooth. Transfer to a bowl, stir in ground walnuts. Let cool completely. Store in refrigerator for up to 2 weeks.