01 - Preheat oven to 350°F (175°C). Grease a 9-inch tart pan with removable bottom.
02 - In a large bowl, combine all-purpose flour, granulated sugar, salt, and unsalted butter. Rub butter into dry ingredients until mixture forms coarse crumbs. Add egg yolk and 2 tablespoons cold water, stirring to form a dough. Add an additional tablespoon of water as needed until dough comes together.
03 - Press dough evenly and firmly into the tart pan. Chill in refrigerator for 20 minutes to set the crust.
04 - Line chilled crust with parchment paper and fill with baking beans or pie weights. Bake for 15 minutes, remove weights and parchment, then bake for an additional 5 minutes or until crust is golden. Allow to cool completely.
05 - In a mixing bowl, whisk mascarpone cheese, powdered sugar, vanilla extract, and lemon zest if using until smooth. In a separate bowl, whisk heavy cream until soft peaks form. Gently fold whipped cream into mascarpone mixture until fully incorporated.
06 - Spread mascarpone filling evenly into the cooled tart shell. Arrange strawberries, blueberries, kiwi, raspberries, and any additional seasonal fruit in a decorative pattern on top of the filling.
07 - Warm apricot jam with water in a small saucepan until liquefied. Using a pastry brush, lightly brush glaze over arranged fruit for added shine, if desired.
08 - Refrigerate assembled tart for at least 30 minutes to set before slicing and serving.