Heat the oven to 350°F. Mix the all-purpose flour, powdered sugar, and fine salt, then work in the cold butter until crumbly. Add the heavy cream and vanilla extract and mix until the dough just comes together.
Press the dough evenly over the bottom and sides of a 9-inch tart pan. Chill for 20 minutes, prick the base all over, and bake for 22 to 26 minutes until golden. Cool completely.
Whisk 1/2 cup of the whole milk with the cornstarch and all-purpose flour until smooth. Add the remaining milk, granulated sugar, and fine salt, then cook over medium heat while whisking until thick and bubbling.
Cook the pastry cream for 1 minute more, remove it from the heat, and whisk in the butter and vanilla extract. Transfer to a shallow bowl, press parchment directly on the surface, and chill until cold.
Whisk the chilled pastry cream until smooth and spread it evenly in the cooled tart shell. Arrange the strawberries, blueberries, and kiwi over the top.
Warm the apricot preserves with the water until fluid, then brush a thin layer over the fruit. Chill the tart for 30 minutes before serving.
