Step 1

Preheat oven to 400°F. Lightly spray a jelly roll pan with cooking spray. Place thawed sheets of puff pastry on a lightly floured surface side by side, allowing short sides to overlap just a bit, and press overlap with fingertips to adhere them. Using a rolling pin, roll combined sheets to a 18- x 13-inch rectangle. Carefully transfer dough to prepared jelly roll pan, and fit into pan, lightly pressing dough up sides of pan. Use a fork to prick holes in dough all over bottom of pan, but not on sides.

Step 2

Whisk together egg and 1 tablespoon of the heavy cream in a small bowl. Lightly brush egg mixture over dough, and sprinkle evenly with 1 tablespoon of the granulated sugar. Bake in preheated oven until lightly golden brown, 10 to 12 minutes. Remove from oven, and allow to cool slightly.

Step 3

Meanwhile, slice berries, peaches, and plums; place each in a separate bowl. Add 1 tablespoon of lemon juice and 1 teaspoon of sugar to each of the 3 bowls of fruit; toss to coat. Let stand at least 5 minutes.

Step 4

Combine marzipan, mascarpone cheese, vanilla extract, and 1/4 cup of the sugar in a food processor. Process until smooth, scraping down sides as needed.

Step 5

Beat together remaining 1 1/2 cups heavy cream and 2 tablespoons sugar with an electric mixer on high speed until stiff peaks form, about 3 minutes. Gently fold mascarpone mixture into whipped cream until combined.

Step 6

Assemble the slab pie: Spread cream mixture over base of pastry crust, spreading gently to edges. Arrange fruits in a single layer over cream mixture. Sprinkle with lemon zest and mint; drizzle with honey. Cut into squares, and serve immediately.