Bring water and lemon juice to a simmer and then add chopped prunes. Simmer with lid off for 10-15 minutes. Cool and put through a food processor. Add nuts.
Spray a square baking pan with non-stick spray. Combine dry ingredients and mix well. Lightly beat egg and butter, add to oat mixture. Press 2 cups of mixture into prepared pan, spread prune filling over base. Sprinkle remaining oat mixture over top. Bake at 180 C for 20 minutes, until brown.
Cool and cut.