Prepare the Pastry: In a food processor, blend cold butter and flour until the mixture resembles coarse crumbs. Add sour cream and pulse until a dough ball forms. Chill for 20 minutes to firm up.

Roll Out Dough: Shape the chilled dough into a disk and roll it out to fit a 25 cm tart tin. Poke holes with a fork to prevent bubbling, then chill for another hour.

Blind Bake: Preheat your oven to 200°C (390°F). Line the pastry with foil, fill with sugar or baking beads, and bake for 15 minutes. Remove the foil and bake for another 5 minutes until golden brown.

Make the Filling: In a mixing bowl, blend ricotta and mascarpone until smooth and creamy. Add sugar, eggs, egg yolks, lemon rind, and lemon juice, mixing until fluffy and well combined.

Bake Tart: Lower the oven temperature to 170°C (340°F). Pour the filling into the baked pastry shell, smoothing it out gently. Bake for 30 minutes, ensuring the center retains a slight wobble. Cool before refrigerating for a firmer texture.

Prepare Candied Lemons (Optional): In a saucepan, cook water and castor sugar over medium heat until a light golden syrup forms. Add thin lemon slices, allowing them to cook and get coated. Remove from heat and let cool.

Finish Tart: Once the tart has cooled completely, remove it from the tin and layer with candied lemon slices if made. Serve chilled for a refreshing treat.