How to Make It
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.
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.
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.
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.
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.
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.