Step by step

Grate the zest of half a lemon and add to the mixture

Tips
If you like, you can add some pine nuts and a little orange essence to the filling. Moreover, if you make it in advance, store it in the fridge: it will last longer and will still be excellent.
Trivia
Probably all sweet shortcrust tarts, and specifically the ricotta tart along with its close Neapolitan cousins, is native to the area of the Gulf of Naples. There are even some legends that link the origin of these desserts to the myth of the mermaid Partenope.