Heat the oven to 375°F. Beat 1 egg and water in a small bowl with a fork or whisk.
Mix the sausage, stuffing, remaining egg, onion and mushrooms in a large bowl.
Unfold the pastry sheet on a lightly floured surface. Cut slits 1-inch apart from the outer edge to the fold mark on each side of the pastry sheet. Spoon the sausage mixture down the center of the pastry. Starting at one end, fold the pastry strips over the sausage mixture, alternating sides, to cover the sausage mixture. Place the pastry onto a baking sheet. Brush the pastry with the egg mixture.
Bake for 35 minutes or until the pastry is golden brown and the sausage is cooked through. Let the pastry cool on the baking sheet on a wire rack for 10 minutes.
Tip: Serves 8 as an appetizer.