Preheat oven to 350°F (175°C). In a food processor, pulse flour and powdered sugar. Add cold butter and pulse until mixture resembles coarse crumbs. Add egg yolk and pulse until dough just comes together.

Press dough evenly into a 9-inch tart pan with removable bottom, bringing it up the sides. Prick the bottom all over with a fork. Freeze for 15 minutes.

Line the crust with parchment paper and fill with pie weights or dried beans. Bake for 20 minutes, then remove weights and parchment. Bake for another 10-12 minutes until golden brown. Cool completely on a wire rack.

In a medium saucepan, whisk together heavy cream, milk, sugar, and cornstarch until smooth. Cook over medium heat, stirring constantly, until mixture thickens and bubbles, about 5-7 minutes.

In a separate bowl, whisk the egg yolks. Slowly pour 1/2 cup of the hot cream mixture into the yolks while whisking constantly to temper them. Pour the egg mixture back into the saucepan.

Continue cooking over medium-low heat, stirring constantly, until the cream is very thick and coats the back of a spoon, about 2-3 minutes. Remove from heat and stir in lemon juice, lemon zest, and butter until smooth.

Pour the lemon cream through a fine-mesh sieve into the cooled tart shell, smoothing the top with a spatula. Press plastic wrap directly onto the surface to prevent a skin from forming.

Refrigerate for at least 4 hours or overnight until completely set and firm. Just before serving, arrange fresh blueberries on top in a single layer or decorative pattern.

Brush the blueberries lightly with warmed apricot jam for a glossy finish. Remove the tart from the pan, slice with a sharp knife wiped clean between cuts, and serve chilled.