01 - Preheat oven to 350°F. Grease a 9-inch tart pan with a removable bottom.
02 - In a large bowl, combine flour, sugar, salt, and butter. Work the butter into the dry ingredients with fingertips or a pastry cutter until mixture resembles coarse crumbs. Add egg yolk and 2 tablespoons cold water, mixing to form a dough. Add an additional tablespoon of water if the dough is too dry.
03 - Press the dough evenly into the tart pan and refrigerate for 20 minutes to firm up.
04 - Place parchment paper over the dough and fill with baking beans or pie weights. Bake for 15 minutes. Remove weights and parchment, then bake an additional 5 minutes or until the crust is golden. Cool crust completely.
05 - In a bowl, whisk mascarpone, powdered sugar, vanilla extract, and lemon zest if using, until smooth. In a separate bowl, whip heavy cream to soft peaks. Gently fold whipped cream into mascarpone mixture until combined.
06 - Spread the mascarpone filling evenly over the cooled tart crust.
07 - Arrange sliced strawberries, blueberries, kiwi, raspberries, and any additional seasonal fruits attractively over the filling.
08 - Warm apricot jam with water until smooth. Using a pastry brush, lightly brush the glaze over the fresh fruit for a glossy finish.
09 - Refrigerate the tart for at least 30 minutes to allow the filling to set. Slice and serve chilled.