Preheat oven to 400°. Cut off the ends of the bread loaves and, using a long serrated knife, hollow out the loaves and place the inside of the loaves into a food processor. Process the bread to fine crumbs and reserve.
Heat the sausage in a large skillet over medium heat, stirring, just until browned. Add the beef and onion. Cook until the beef is lightly browned, stirring to break up any large pieces. Cool the meat mixture briefly, then add the egg, mustard, reserved bread crumbs and seasoning.
Spoon the mixture into the hollowed out bread, packing tightly.
Brush the loaves with the melted butter, wrap in separate pieces of foil, leaving a large opening at the top of each loaf.
Place the loaves on a cookie sheet and bake for 15-20 minutes. Serve immediately, or, if serving at a later time, cool in the refrigerator. When cool, wrap the loaves tightly in foil and freeze. To serve, partially thaw the loaves in the refrigerator before slicing into 3/4 inch pieces. Place the partially thawed slices on a cookie sheet and heat at 400°, turning once, until golden brown.
Thin the Dijon mustard with cream until it reaches a consistency of heavy cream. Serve the bread slices with a small portion of the mustard mixture. Makes 40-50 slices.