Any seasonal berry works with this dessert.
Preheat oven to 400°. Let puff pastry sheets stand at room temperature 20 minutes.
Meanwhile, stir together blackberries, granulated sugar, lemon juice, and 1 Tbsp. bourbon in small bowl. Cover and chill until ready to use.
Unfold pastry sheets onto a lightly floured surface, and cut into 3 strips along fold marks. Cut each strip into 4 (2- x 3-inch) rectangles, discarding scraps. Place rectangles in a single layer on a parchment paper-lined baking sheet. Place parchment paper over pastry rectangles, and top with a second baking sheet to weight the dough.
Bake weighted pastry rectangles at 400° for 25 minutes or until golden brown. Remove and discard top layer of parchment paper. Cool completely on wire rack (about 15 minutes). Using a small knife, separate each rectangle into 2 thinner pieces.
Beat cream and eggnog at medium-high speed with an electric mixer until foamy; gradually add powdered sugar and nutmeg, beating until soft peaks form. Stir in remaining 1/2 Tbsp. bourbon.
Spoon mixture into a zip-top plastic freezer bag. Twist end of bag to close loosely. Snip 1 corner of bag to make a small hole (about 1/2 inch).
Place a pastry rectangle on a dessert plate. Pipe small mounds (about 1/2 inch high) of eggnog mixture onto pastry rectangle; top with second pastry rectangle. Pipe another layer of eggnog mixture mounds, and top with third pastry rectangle. Dust with powdered sugar. Repeat with remaining pastry rectangles and eggnog mixture. Serve immediately with blackberry mixture.