Put the flour in the bowl of an electric mixer. On slow, mix in the butter until it looks like very coarse meal. Mix in the sour cream until it masses together to form a dough. Pat into a rectangle, wrap in plastic and refrigerate for 1 hour or overnight.
Coarsely chop the prunes. In a small saucepan, cover the prunes with the prune juice and sugar. Bring to a simmer for 3 minutes. Place in a food processor with the vanilla and puree to a paste. Mix in the raisins, cherries and walnuts. Refrigerate for 1 hour. Preheat the oven to 400 degrees. On a lightly floured surface, roll the dough out into a square 16 by16 inches. Place the filling in a log running along the bottom and roll up. Pinch the ends closed. Place on a parchment lined cookie sheet, seam side down. Mix the egg and water together to form a wash and paint the strudel. Bake for 30 minutes until crisp and golden. Let cool on a rack, then slice into 1 inch pieces.
This was my Grandma Bessie's baking specialty. I haven't had this since she died but my brothers and I talk of it often, trying to re-construct what was in it. I re-discovered it at the Golden Key in Brighton Beach. Can be made on a rainy day with items in the pantry