Preheat the oven to 375°F.

Let the frozen pastry thaw 20 minutes countertop, separate the dough so you defrost just the one sheet and refreeze the other. You will know the dough is ready to work with when it unfolds easily. Lay the pastry out flat and where it has folded, press out creases as needed. Cut pastry crosswise in half. Wrap each piece of sausage in pastry, trimming pastry as necessary and sealing edges with egg mixture. Use pastry trimmings and small cookie cutters to decorate the Sausage En Croûte. Brush with more egg and place on an ungreased baking sheet. (This may be made ahead to this step, loosely covered and refrigerated.) Bake for 35 to 40 minutes or until browned and puffed. Remove from oven and cool slightly.

To make the mustard, combine the dry mustard and white vinegar. Let the mixture sit overnight. The next day add eggs, sugar and salt and heat to the desired consistency. Refrigerate until ready to serve. Serve with the Sausage En Croûte.