Preheat oven to 375 degrees F. Unfold puff pastry sheet; roll into a 15x10-inch rectangle. Cut rectangle crosswise into ten 1-1/2-inch-wide strips; cut strips crosswise into thirds. Place pieces about 1 inch apart on two ungreased baking sheets. Combine egg and water. Brush egg mixture onto rectangles.
Bake for 8 to 10 minutes or till light brown. Remove from oven. Cool completely on a wire rack. Store in an airtight container at room temperature up to 3 days or freeze up to 3 months.
For topping, stir together sour cream, the 2 tablespoons herbs, horseradish, milk, and garlic. Cover and chill up to 24 hours.
To serve, spread about 2 teaspoons of the sour cream mixture on each pastry rectangle, leaving a small border. Top with a thin slice of beef tenderloin, arranging accordion-style. Sprinkle with cracked pepper. Garnish with fresh herbs. Arrange on a platter. Makes 30 sandwiches.