Preheat oven to 400°F.
On a silicone mat or sheet of parchment paper, roll out the puff pastry to a roughly 12x10-inch rectangle. Use the tines of a fork to prick the inside of the pastry several times, so it poofs up less as it bakes, then use the tines flat to press a decorative pattern of lines around the outside edge of the rectangle.
1 sheet puff pastry
Combine the egg and milk or water in a small bowl, then brush this mixture lightly around the outside edge of the puff pastry. (You'll have extra egg wash, which you can discard or save for another project.) Bake for 18-20 minutes, or until golden brown. Set aside to cool.
1 egg, 2 teaspoons milk or water
While the pastry bakes, prepare the topping. In a large bowl, combine the mascarpone, whipped cream, and sugar. Whip with a spatula until evenly combined. Set aside.
4 ounces mascarpone cheese, 4 ounces whipped cream, 2 Tablespoons granulated sugar
Wash and slice the berries or fruit as desired.
2 cups berries or sliced fruit
Once the puff pastry has cooled all the way to room temperature, spread the cream on top in an even layer, then arrange berries on top. Add mint leaves or other garnish if desired. Refrigerate until ready to slice and serve.
fresh mint leaves