1. Beat cream cheese at medium speed with a heavy-duty electric stand mixer until smooth. Stir in mushrooms and next 4 ingredients. Cover and chill 1 to 24 hours.
2. Preheat oven to 400°. Whisk together egg and 1 Tbsp. water in a small bowl. Roll 1 puff pastry sheet into a 16- x 10-inch rectangle on a lightly floured surface. Cut pastry in half lengthwise. Spread 1/2 cup cream cheese mixture down center of each rectangle; brush edges with egg mixture. Fold each pastry half lengthwise over filling, and pinch edges to seal. Cut pastries into 10 pieces each, and place on a parchment paper-lined baking sheet. Repeat procedure with remaining puff pastry sheet, egg mixture, and cream cheese mixture.
3. Brush remaining egg mixture over tops of pastry pieces; sprinkle with pepper.
4. Bake at 400° for 20 to 25 minutes or until browned. Serve immediately.
*2 (5.2-oz.) packages buttery garlic-and-herb spreadable cheese may be substituted.
Note: To make ahead, prepare recipe as directed through Step 3. Freeze pieces on baking sheet until firm (about 1 hour), and transfer to zip-top plastic freezer bags. Freeze up to 1 month. To bake, place frozen puffs on parchment paper-lined baking sheets, and proceed as directed in Step 4.