01 - Preheat oven to 350°F. Lightly grease a 9-inch tart pan with a removable bottom.
02 - In a large mixing bowl, blend together flour, granulated sugar, salt, and cubed chilled butter. Rub butter into dry ingredients by hand or use a pastry blender until the mixture resembles coarse sand.
03 - Incorporate egg yolk and 2 tablespoons cold water. Mix gently to form a dough, adding an additional tablespoon of water if needed until the mixture comes together.
04 - Press dough evenly across the bottom and up the sides of the prepared tart pan. Chill in the refrigerator for 20 minutes to allow the dough to set.
05 - Line the chilled crust with parchment paper and fill with baking beans or pie weights. Bake for 15 minutes, carefully remove weights and parchment, and bake an additional 5 minutes until the shell is golden. Cool completely before assembling.
06 - In a clean bowl, whisk mascarpone cheese, powdered sugar, vanilla extract, and lemon zest (if using) until smooth. In a separate bowl, whip heavy cream to soft peaks, then gently fold whipped cream into the mascarpone mixture until combined.
07 - Spread prepared mascarpone filling evenly into the cooled tart shell using a spatula.
08 - Decoratively arrange strawberries, blueberries, kiwi, raspberries, and other berries or seasonal fruits over the mascarpone filling.
09 - Warm apricot jam and water together until liquid, then brush gently over arranged fruit using a pastry brush to create a glossy finish.
10 - Refrigerate tart for at least 30 minutes to allow filling to set before slicing and serving.