In a food processor mix together all the cheesecake base ingredients until combined.
Put the mixture into a cake tin covered with cling film, and use your fingers or the back of a spoon to spread it evenly, pressing it down in the tin to form the base. Put the tin into the fridge to set for about 30 min.
In the meantime prepare the filling. Put the ricotta and sugar in a large bowl and mix energetically with a spatula or hand blender until smooth and creamy.
Add in the orange zest & lemon, vanilla and pistachio if using, then fold the nutella in. Gently mix by hand until combined.
Remove the cheesecake tin from the fridge. Pour the filling over the base and spread evenly.
Return to the fridge until set and ready to serve (rest at least for 4 hours).
Makes one large 9-inch cheesecake or 8 small individual ones. For the classic cheesecake base mix 15 finely crumbled digestive biscuits, 2+½ tbsp melted butter and 1 tbsp sugar.