In a food processor, pulse together the flour, powdered sugar, and salt. Add the cold butter cubes and pulse 8-10 times until the mixture resembles coarse crumbs with pea-sized butter pieces remaining.
Add the egg yolk and pulse to combine. Add ice cold water one tablespoon at a time, pulsing after each addition, just until the dough begins to clump together when pressed. Do not over-mix.
Turn the dough out onto a lightly floured surface, press it gently into a flat disk, and wrap tightly in plastic wrap. Refrigerate for at least 30 minutes to relax the gluten and firm up the butter.
On a lightly floured surface, roll the chilled dough into a 12-inch circle about 1/8 inch thick. Drape over the tart pan, press into the bottom and fluted sides, and trim the excess. Dock the bottom all over with a fork.
Place the lined tart pan in the freezer for 15 minutes while you preheat the oven to 375°F (190°C). Chilling prevents shrinkage during blind baking.
Line the frozen tart shell with parchment paper and fill with pie weights or dried beans. Bake for 15 minutes, then carefully remove the weights and parchment and bake for another 8-10 minutes until the crust is pale golden and the bottom looks dry.
Remove the tart shell from the oven and allow it to cool completely on a wire rack before filling. A warm shell will melt the mascarpone cream.
Beat the room-temperature mascarpone with a hand or stand mixer on medium speed for 1 minute until smooth. Add the powdered sugar, vanilla extract, and lemon zest, and mix until well combined.
In a separate chilled bowl, whip the cold heavy cream to stiff peaks. Gently fold the whipped cream into the mascarpone mixture in two additions using a rubber spatula, until just combined and no white streaks remain.
Spoon the mascarpone cream into the cooled tart shell and spread it into an even layer with an offset spatula. Refrigerate for at least 30 minutes to allow the filling to firm up before topping with fruit.
Hull and slice the strawberries, peel and slice the kiwis, rinse and dry the berries, and remove any pith from the mandarin segments. Arrange the fruit on the chilled filling in concentric rings or your desired pattern, pressing each piece gently into the cream.
Melt the apricot jam with the warm water in a small saucepan over low heat until smooth and pourable, about 1-2 minutes. Using a pastry brush, gently dab the warm glaze over all the fruit in a thin, even layer.
Refrigerate the finished tart for at least 30 minutes before serving. Remove the outer tart pan ring, slice with a sharp knife wiping the blade between cuts, and serve cold.