Thaw puff pastry at room temperature for 30 minutes. Heat oven to 375°F.
Mix ONE egg with 1 tablespoon water. Set aside.
Mix together sausage, stuffing, remaining egg, onion, and mushrooms.
Unfold puff pastry onto a lightly floured baking sheet, creating 3 sections (lengthwise). DO NOT separate the sections. Cut 9 strips in the two outer sections, about 1 inch apart, cutting just to the fold line in the pastry. These strips will be criss-crossed over the stuffing.
Spoon sausage mixture in the center of the pastry. Starting at one end, fold the pastry strips over the sausage mixture, alternating sides. Brush with egg mixture.
Bake 35 minutes or until golden. Slice and serve warm.