In a food processor, combine cold cubed butter and plain flour. Pulse until it resembles coarse breadcrumbs, then add sour cream. Continue to pulse until the dough forms a ball. Wrap in plastic wrap and refrigerate for 20 minutes.

Shape the dough into a disk on a lightly floured surface. Roll it out to fit a tart tin, pressing it into the edges. Poke holes in the base and chill in the refrigerator for 1 hour.

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

In a mixing bowl, blend fresh ricotta cheese and mascarpone until smooth. Mix in sugar, whole eggs, egg yolks, lemon rind, and lemon juice until fluffy and well combined.

Reduce the oven temperature to 170°C (340°F). Pour the filling into the baked pastry shell and bake for about 30 minutes until there's a slight wobble in the center. Cool in the tin before refrigerating for at least 2 hours.

Combine water and castor sugar in a saucepan over medium heat until the sugar dissolves. Add lemon slices, cooking until translucent. Allow to cool and set.

Carefully remove the tart from the tin. If using, layer candied lemons on top for garnish. Serve chilled.