For the limoncello, pour a little of the alcohol into a pan, add the sugar and heat gently, stirring occasionally, until the sugar has dissolved.
Remove from the heat and add the lemon juice and zest. Stir in the remaining alcohol and allow to cool. Pour into a clean bottle, then place into the fridge or freezer to chill thoroughly.
For the biscotti, preheat the oven to 180C/350F/Gas 4. Line a baking sheet with baking parchment.
Mix the flour, sugar and baking powder together in a large bowl. Add half the beaten eggs and mix well, then add half of what's left and mix again. Add the last quarter a little at a time until the dough takes shape but isn't too wet (you may not need to use all of the eggs). Add the fruit, nuts and lemon zest and mix well.
Divide the dough into six pieces. With wet hands, roll each piece into a sausage shape about 5cm/2in wide and place well apart on the baking sheet. Lightly flatten the 'sausages' and bake for about 20 minutes, or until golden-brown. Remove from the oven and leave to cool and harden for ten minutes.
With a serrated knife, cut the 'sausages' on an angle into 0.5cm/¼in slices and lay these on the baking sheet. Return to the oven and bake for eight minutes, then turn the slices over and cook for a further 10-15 minutes, or until they are a pale golden colour. Remove from the oven and cool on wire racks. When completely cold, the biscotti can be stored in airtight jars for a week or more.
To serve, pour the limoncello into a chilled shot glass and serve with the biscotti.