In a medium bowl, blend together the butter, cream cheese and salt until smooth. Stir in the flour, until the mixture is evenly crumbly.
Sprinkle the milk over the flour mixture one tablespoon at a time, stirring with a fork until the dough is moistened enough to be formed into a ball. Press dough together, and knead for just a couple of turns. Flatten the ball slightly, wrap and refrigerate.
Heat the oil in a large skillet over medium-high heat.
Add the ground beef and onion, and cook until the beef is no longer pink.
Drain off fat, and set aside to cool.
Transfer the meat mixture to a bowl, and mix in the salt, egg, sour cream, blue cheese, olives and walnuts until well blended.
Preheat the oven to 425 degrees F (220 degrees C).
On a sheet of waxed paper, or between two sheets, roll the pastry out into a rectangle about 10x12 inches.
Place the meat mixture down the center of the pastry, then bring the sides up around the filling, pinching together to enclose the filling completely. Use the waxed paper to pick up the roll, and roll it onto a baking sheet so that it sits with the seam on the bottom.
Bake for 25 minutes in the preheated oven, or until the pastry is a nice golden brown.
