Blend fresh blueberries in a blender until smooth. Pour into a saucepan with caster sugar and water. Cook on medium heat for about 10 minutes, stirring occasionally, until thickened into a syrup. Remove from heat and let it cool.

Combine plain flour, icing sugar, ground ginger (if using), and a pinch of salt in a food processor. Add cold unsalted butter and process until resembling breadcrumbs. Mix in egg yolk and a splash of cold water until a dough forms. Wrap in plastic wrap and chill for 20 minutes.

Remove chilled dough and roll out on a floured surface to about 4mm thickness. Line a tart tin and trim excess pastry. Chill for another 10 minutes.

Preheat oven to 190°C (375°F). Line the pastry with baking paper and fill with beans/rice. Bake for 15 minutes, then remove paper and bake for an additional 5-10 minutes until golden.

In a mixing bowl, whisk together egg yolks, condensed milk, lemon juice, lemon zest, and salt until smooth.

Spread lemon filling into cooled tart shell. Swirl the cooled blueberry puree on top. Bake in preheated oven at 170°C (340°F) for about 15 minutes.

Let cool to room temperature. Refrigerate for a few hours before serving. Garnish with extra blueberries and lemon slices.