Combine flour, powdered sugar, and salt in a bowl. Add the cold butter cubes and rub into the flour with your fingertips until the mixture resembles rough breadcrumbs with some larger pieces still visible - this creates flakiness. Add the egg yolk and 2 tablespoons of ice water and mix with a fork until the dough just comes together. Add the third tablespoon of water only if needed. The dough should hold together when pressed but not be sticky.

Shape into a flat disc, wrap in plastic wrap, and refrigerate for at least 30 minutes. Cold pastry is the difference between a crisp base and a soggy one - don't skip this step.

Preheat oven to 375F. On a lightly floured surface, roll the pastry out to about 3mm thickness and large enough to line a 9-inch fluted tart tin with a removable base. Carefully drape the pastry over the tin and press it into the flutes without stretching. Trim the excess from the top. Prick the base all over with a fork.

Line the pastry with parchment paper and fill with baking weights or dried beans. Bake for 15 minutes, then remove the weights and paper and bake for a further 8-10 minutes until the base is dry and just starting to turn golden. Remove from the oven and allow to cool completely in the tin.

Whisk the egg yolks, sugar, and cornstarch together in a medium bowl until pale and smooth. Heat the milk and cream in a saucepan over medium heat until just steaming - not boiling. If using a vanilla bean, add it now and let it infuse for 5 minutes then remove.

Pour the hot milk mixture slowly into the egg mixture, whisking constantly as you pour. This tempers the eggs so they don't scramble. Pour everything back into the saucepan and cook over medium heat, stirring constantly with a wooden spoon or spatula, until the custard thickens enough to coat the back of a spoon - about 5-7 minutes. It will look thin and then suddenly thicken quite quickly, so don't walk away.

Remove from heat, stir in the vanilla extract if using, and the cold butter. The butter gives the custard a beautiful glossy finish. Pour into a bowl, press plastic wrap directly onto the surface to prevent a skin forming, and refrigerate until completely cold - at least 2 hours.

Toss the sliced rhubarb with 2 tablespoons of sugar and leave for 15 minutes. This draws out some of the moisture and softens the rhubarb slightly so it bakes evenly. Slice the strawberries to roughly the same thickness as the rhubarb so everything cooks at the same rate.

Spread the cold custard evenly into the cooled tart shell. It should fill it to about 3/4 full. Arrange the rhubarb and strawberry slices on top of the custard in whatever pattern appeals to you - concentric circles look the most polished, but a loose overlapping arrangement looks equally beautiful and more relaxed. The key is covering the custard completely so every slice gets fruit.

Bake at 350F for 20-25 minutes until the fruit is softened and the custard is just set with a slight wobble in the center. It will firm up further as it cools. Remove from the oven and allow to cool completely in the tin.

Warm the apricot jam with a tablespoon of water in a small pan until it becomes liquid. Strain out any large pieces. Brush the glaze gently over the fruit topping using a pastry brush. This gives the tart that professional patisserie finish - glossy, jewel-toned, and beautiful. It also preserves the fruit and stops it drying out.

Refrigerate for at least 1 hour before serving. Remove from the tin just before you bring it to the table.