01 - Preheat oven to 350°F. Lightly grease a 9-inch tart pan with removable base.
02 - In a large bowl, combine flour, sugar, salt, and chilled butter. Work butter into flour by hand or with a pastry blender until mixture forms coarse crumbs. Add egg yolk and 2 tablespoons cold water; gently mix to form a dough, adding another tablespoon of water if needed.
03 - Press dough evenly into the base and up the sides of the prepared pan. Chill the shell in the refrigerator for 20 minutes to firm.
04 - Place parchment paper over the chilled dough and fill with baking beans or pie weights. Bake for 15 minutes. Remove the weights and parchment, then bake for an additional 5 minutes until golden. Allow crust to cool completely.
05 - In a medium bowl, whisk mascarpone, powdered sugar, vanilla, and lemon zest (if using) until smooth. In a separate bowl, whip heavy cream to soft peaks. Gently fold the whipped cream into the mascarpone mixture until fully combined.
06 - Evenly spread mascarpone filling into the cooled tart shell, smoothing the top.
07 - Decoratively arrange strawberries, blueberries, kiwi, raspberries, and any additional fruit as desired over the mascarpone layer.
08 - Gently warm apricot jam with water in a small saucepan until liquid. Using a pastry brush, lightly coat the fruit for an attractive shine.
09 - Refrigerate assembled tart for at least 30 minutes to set the filling before slicing and serving.