Cook first 5 ingredients in a large skillet over medium-high heat, stirring until beef crumbles and is no longer pink; drain. Stir in ketchup and Worcestershire sauce; cool.
Unfold 1 piecrust, and place on a lightly greased baking sheet. Spread mustard evenly over crust. Stir together meat mixture and 2 cups cheese; spoon onto center of crust, leaving a 2-inch border.
Unfold remaining piecrust, and cut out a jack-o'-lantern face, reserving pastry cutouts to use as a stem. Place crust over meat mixture; crimp edges of crust, and fold under. Place stem on top of jack-o'-lantern face.
Whisk together 2 tablespoons water, egg, and 1 drop each of red and yellow food coloring; brush over crust.
Bake at 425° for 20 minutes; remove from oven, and brush again with egg mixture. Fill eyes, nose, and mouth with remaining 1 cup cheese. Bake 5 to 10 more minutes or until golden brown.