Preheat the oven to 375°F (190°C).

Line a small baking pan with parchment paper. Place the quartered strawberries in the pan, sprinkle with powdered sugar, and toss well. Roast for 20 minutes, or until reduced and just caramelized. Let cool at room temperature.

Lay the thawed puff pastry on a work surface and cut to fit your baking sheet. Line the sheet with parchment paper and lay the dough on top. With a sharp knife, lightly score a rectangle inside the pastry, leaving a 1¼-inch/3 cm border. Pierce the inner rectangle with a fork. Brush the outer border with a beaten egg.

Bake for 25 minutes, or until deeply golden and puffed. Let cool completely.

In the bowl of a stand mixer fitted with the whisk attachment, combine the mascarpone, heavy cream, and powdered sugar. Beat until stiff, smooth peaks form.

To assemble, place the baked pastry on a serving platter. Carefully scrape out the center rectangle with a fork and reserve the crumbs. Spread the mascarpone cream evenly over the base. Top with roasted strawberries and garnish with the reserved pastry crumbs.

Assemble the dessert just before serving for best texture.