01 - Preheat oven to 350°F. Lightly grease a 9-inch tart pan with removable bottom.
02 - In a large mixing bowl, combine flour, granulated sugar, salt, and butter. Rub or cut butter into dry ingredients until mixture resembles coarse crumbs. Add egg yolk and 2 tablespoons cold water, mixing gently to form a dough. Add an additional tablespoon cold water if needed to bring dough together.
03 - Press dough evenly across the base and sides of the prepared tart pan. Refrigerate for 20 minutes to firm the dough.
04 - Line chilled crust with parchment paper and fill with baking beans or pie weights. Bake 15 minutes, remove weights and paper, and continue baking 5 minutes until golden. Cool completely in pan.
05 - 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 mascarpone mixture until fully combined.
06 - Spread mascarpone filling evenly over cooled tart crust, smoothing the surface with a spatula.
07 - Decoratively arrange strawberries, blueberries, kiwi, raspberries, and any additional fruits over the mascarpone layer.
08 - In a small saucepan, gently heat apricot jam with water until fluid. Brush glaze lightly over the arranged fruit for a glossy finish.
09 - Refrigerate tart for at least 30 minutes to firm up before slicing and serving.