Preheat oven to 425°. In a large skillet, heat butter over medium-high heat. Add mushrooms and onion; cook and stir 5-7 minutes or until tender and liquid is evaporated. Remove from pan; cool completely., Drain beef, discarding sauce or saving for another use. Coarsely chop beef. In a small bowl, whisk egg and water., Unfold one sheet of puff pastry. On a lightly floured surface, roll pastry into a 15x9-in. rectangle; cut crosswise into thirds, making three 5-in.-wide rectangles., Place a cheese slice on one half of each rectangle, trimming cheese to fit. Top each with a rounded tablespoon mushroom mixture and scant 3 tablespoons beef. Lightly brush edges of pastry with egg mixture. Fold pastry over filling; press edges with a fork to seal. Transfer to a parchment-lined baking sheet. Repeat with remaining pastry sheet and filling., Brush tops with egg mixture. Bake 17-20 minutes or until golden brown. Serve warm. Freeze option: Freeze unbaked pastries on a waxed paper-lined baking sheet until firm. Transfer to a resealable plastic freezer bag; return to freezer. To use, bake frozen pastries on a parchment-lined baking sheet in a preheated 400° oven 20-25 minutes or until golden brown and heated through.