Heat the milk and vanilla bean in a saucepan until scalding.
While the milk is heating, place the sugar and egg yolks in a bowl and whisk until the yolks have lightened in colour. This will take a few minutes of vigorous whisking. Whisk in the cornstarch and salt.
Remove the vanilla bean from the heated milk and, using the back of a knife, scrape the seeds back into the milk.
Slowly drizzle the hot milk into the yolk mixture while continuing to whisk. If you add the hot milk too quickly the eggs will curdle and your pastry cream will come out lumpy.
Once all the milk has been added, transfer the mixture back to the saucepan and place over medium heat, Whisking constantly, bring the mixture to a boil, and continue cooking for five minutes more, whisking the entire time.
Remove from heat. Immediately strain the pastry cream through a fine mesh strainer to remove any lumps. Add the butter and whisk until smooth; or, if you want your pastry cream even smoother, use an immersion blender.
Cover the pastry cream and refrigerate for two to three hours, until set.
To assemble tart:
Use a spatula to slightly break up the cold pastry cream. Fill the tart shell with pastry cream to just slightly below the rim, spreading it out smoothly with a knife or a small offset spatula.
Arrange the fresh berries or other fruit in a pattern on top.
In the microwave or over the stove, gently melt the apricot jam with the water –without letting it come to a boil –and brush it generously over the top of the fruit. Garnish with fresh lemon zest. If not serving immediately, refrigerate until ready to serve.
Storage: This fruit tart will keep in the refrigerator for up to three days.