In a food processor, pulse together the cold butter and all-purpose flour until the mixture resembles coarse breadcrumbs, about 1–2 minutes. Add the sour cream and pulse until a dough forms. Shape the dough into a disk, wrap it in plastic wrap, and refrigerate for 20 minutes.

Once the dough has chilled, roll it out on a floured surface to fit a 25 cm (10”) tart pan. Carefully transfer the rolled dough into the pan and poke holes in the bottom with a fork. Chill the tart shell in the refrigerator for 60 minutes.

Preheat your oven to 200°C (390°F). Line the chilled tart shell with parchment paper, filling it with pie weights or sugar. Bake for 15 minutes, then carefully remove the parchment and weights, returning it to the oven for an additional 5 minutes until lightly golden. Allow the tart shell to cool completely.

Lower the oven temperature to 170°C (340°F). In a mixing bowl, blend the fresh ricotta and mascarpone until smooth. Gradually add sugar, whole eggs, egg yolks, finely grated lemon rind, and fresh lemon juice; blend until fully combined.

Pour the creamy filling into the cooled tart shell, spreading it evenly. Bake in the preheated oven for 30 minutes, or until the center wobbles gently when nudged.

Allow the tart to cool for about 1 hour at room temperature, then refrigerate until fully set, at least 2 hours, or preferably overnight.

In a skillet, combine castor sugar and water, cooking over medium heat until it turns golden and syrupy, about 5–7 minutes. Thinly slice the lemon and add to the syrup, cooking gently until coated and translucent.

Once the tart is set and chilled, remove it from the pan. Top the tart with the caramelized lemon slices, arranging them artfully.