01 - Preheat the oven to 350°F. Grease a 9-inch tart pan with a removable bottom.
02 - In a large bowl, combine flour, granulated sugar, salt, and butter. Rub butter into dry ingredients until mixture resembles coarse crumbs. Add egg yolk and 2 tablespoons cold water. Mix just until dough forms, adding up to 1 extra tablespoon water if necessary.
03 - Press dough evenly into the bottom and sides of prepared tart pan. Refrigerate for 20 minutes to firm up.
04 - Line tart shell 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 until golden. Allow shell to cool completely.
05 - In a bowl, whisk mascarpone, powdered sugar, vanilla, and lemon zest (if using) until creamy and smooth. In a separate bowl, whip heavy cream to soft peaks, then fold whipped cream gently into mascarpone mixture.
06 - Spread mascarpone filling evenly into the cooled crust, smoothing the top.
07 - Place strawberries, blueberries, kiwi, raspberries, and other fruits decoratively on top of the mascarpone filling.
08 - Gently warm apricot jam with water until liquid. Brush mixture lightly over the arranged fruit for a glossy finish, if desired.
09 - Refrigerate assembled tart for at least 30 minutes to set before slicing and serving.