Melt the butter and cook the onions until transparent, about 5 minutes. Warm the milk in a small saucepan and soak the rolls. In a mixing bowl, combine the onions, drained rolls, pork, veal, parsley, Riesling, salt and pepper. Mix well, cover and refrigerate over night. Preheat the oven to 350F. Defrost the puff pastry. In a food processor combine the flour and butter until the mixture resembles bread crumbs. Add the salted water and process until the mixture clumps. Form mixture into a ball, cover, and refrigerate. Roll the pie dough out to a thickness of 1/4-inch and line a 10-inch pie pan. Spoon the filling into the crust. Roll the puff pastry out thinly and place over the filling, attaching the edges with a little water. Brush the pie with beaten egg. Bake for 1 hour.
