In a large bowl, mix the flour and sugar.

Add in the sourdough starter, softened butter, eggs, and water.

Mix everything together with a spatula or in a stand mixer with the dough hook attachment just until everything is well combined. This is a shaggy and sticky dough. Don't be tempted to add more flour.

Cover the bowl and let it rest for 30 minutes.

Perform a set of stretch and folds, cover, and let it ferment. If the dough is not smooth and elastic after the stretch and folds, I recommend you do one more set of stretch and folds after 30 minutes from the first set. Let the dough bulk ferment in a warm spot.

The bulk fermentation should be done when your dough has almost doubled in size (I go for about 85%-90%, if you are measuring). The dough should be domed on top, have lots of large bubbles, and be light and airy (with a noticeable jiggle when gently shaking the bowl). Fermentation time varies depending on the temperature in your kitchen. It can take from 4 to 20 hours.

Sprinkle some flour on your work surface and place the dough on your counter. Divide the dough into 8 equal pieces and shape each into balls.

Flatten the dough gently with your hands (disc shape).

With the bottom of a small glass (or with your hands), gently press in the middle to create a space for the filling.

Put the tarts on a baking sheet lined with parchment paper. Space them out a bit as they will spread and spring slightly when baked.

Preheat your oven to 325°F (163°C).

In a medium-size bowl, add the mascarpone cheese, vanilla extract, lemon zest, and icing sugar. Mix well with a small whisk. The mixture should have a smooth, creamy texture.

Add about a tablespoon of the cream in the middle of the shaped tarts where you created that hollow in the dough.

I like to close up the tarts by lifting a bit of the dough with two fingers and sticking it on itself. I do this all around the tart. Sort of like the stretch and folds earlier but on a smaller and more gentle scale. This will give the brioche tarts a bit more structure, but it is optional.

Add a few blueberries on top of the cream in each tart. About 6 or 7 depending on the size of your blueberries.

Create an egg wash by whisking together one small egg with a bit of water.

With a pastry brush, gently brush the egg wash on the dough all around (avoiding the centre). This will give that beautiful golden brown colour and a shiny appearance.

Sprinkle a bit of granulated sugar all over the tart. The egg wash will help it stick. This will give a lovely crunch and sparkle to the finished tarts.

Bake the tarts at 325°F (163°C) for about 30 minutes or until golden brown. The mascarpone cream and blueberries will have melted beautifully into the dough.

Leave the blueberry tarts to cool off on the baking sheet for a few minutes before placing them on a wire rack.

Best served warm. If you wish to make these in advance, I recommend warming them up before serving.