Preheat oven to 425 degrees F. In a large bowl, combine flour, dry potatoes, milk, and oil; set aside. (Mixture will stiffen slightly as it stands.)
For filling, in a large skillet, cook ground beef, sausage, and onion over medium heat until meat is brown and onion is tender. Drain off fat. Stir in tomato sauce, tomato paste, olives (if desired), and seasoning mix.
Using floured fingers, press half of the dough onto the bottom and about 1-1/2 inches up the sides of an ungreased 3-quart rectangular baking dish. Spread filling over crust; sprinkle with cheese.
Place the remaining dough on a lightly floured sheet of waxed paper. Sprinkle dough lightly with flour; top with another sheet of waxed paper. Roll dough into a 15x11-inch rectangle; remove top paper. Invert dough onto filling; remove paper. Trim edges as necessary. Turn under edges of top crust and seal to bottom crust. Sprinkle with cornmeal.
Bake, uncovered, for 25 to 30 minutes or until filling is heated through and crust is golden. Let stand for 5 minutes before serving. Makes 12 servings.