01 - Preheat oven to 350°F. Lightly grease a 9-inch tart pan with a removable bottom.
02 - Combine flour, sugar, salt, and chilled butter in a large mixing bowl. Rub butter into flour until mixture forms coarse crumbs. Stir in the egg yolk and 2 tablespoons cold water, mixing gently to form a dough. Add an additional tablespoon of water if needed to bring dough together.
03 - Press the dough evenly into the tart pan, covering the base and sides. Refrigerate the lined pan for 20 minutes to prevent shrinkage.
04 - Line the crust with parchment paper and fill with baking beans or pie weights. Bake for 15 minutes, remove weights and paper, and bake for 5 more minutes until golden. Cool completely on a wire rack.
05 - Whisk mascarpone, powdered sugar, vanilla extract, and lemon zest together in a bowl until smooth. In a separate bowl, whip heavy cream to soft peaks, then gently fold whipped cream into mascarpone mixture until fully incorporated.
06 - Spread the mascarpone mixture evenly into the cooled tart crust using a spatula.
07 - Decoratively place sliced strawberries, blueberries, kiwi, raspberries, and any additional fruit on top of the mascarpone filling.
08 - Heat apricot jam and water until fluid, then brush gently over the arranged fruit for a glossy finish, if desired.
09 - Refrigerate the tart for at least 30 minutes to set the filling before slicing and serving.