01 - Preheat oven to 350°F. Grease a 9-inch tart pan with removable bottom.
02 - In a large bowl, combine flour, granulated sugar, pinch of salt, and chilled butter. Rub butter into flour until the mixture resembles coarse crumbs. Add egg yolk and 2 tablespoons cold water, mixing until a dough forms. If needed, add an additional tablespoon water.
03 - Press dough evenly into the prepared tart pan, covering the bottom and sides. Refrigerate for 20 minutes to chill.
04 - Line the chilled crust with parchment paper and fill with baking beans or pie weights. Bake for 15 minutes. Remove parchment and weights, then bake for an additional 5 minutes or until golden brown. Cool the crust completely on a rack.
05 - In a mixing 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. Fold whipped cream into mascarpone mixture gently until fully combined.
06 - Spread the mascarpone filling evenly into the cooled crust, smoothing the top with a spatula.
07 - Arrange strawberries, blueberries, kiwi, raspberries, and additional fruits decoratively on top of the filling.
08 - In a small saucepan, warm apricot jam with water until liquefied. Brush gently over the fruit to provide a glossy finish.
09 - Refrigerate tart for at least 30 minutes to set before slicing and serving.