Mix the ground almonds and chopped pistachios in a bowl, reserving 3 tbsp of the pistachios to garnish. Stir in the finely grated lemon zest, the 3 tbsp of sugar and 5 tbsp of the melted butter. Heat the oven to 180C/160C fan/gas 4. Brush the inside of a roasting or baking tin (30 x 20cm) with the butter.
Whatever size of filo sheets you have, cut them to fit the tin. Lay one sheet of filo in it, butter it, and keep going until you have 6 sheets of filo. Spoon the nut mixture on top and press down with the back of a spoon, like making a biscuit base, then put 8 filo sheets on the top, again buttering each one. Using a sharp knife, cut a diamond trellis on top, then bake in the oven for 1 hr until golden brown.
Make the syrup by putting the sugar, lemon juice and strips of zest into a saucepan with 250ml water. Bring to a boil, stirring to help the sugar dissolve, then turn the heat down a little and simmer for 10 mins. The mixture will look quite syrupy. Remove the lemon zest and add the rosewater to taste.
While the baklava and the syrup are still warm, pour the syrup carefully over the pastry. Return the baklava to the oven for 5 more mins.
Sprinkle the remaining pistachios over the baklava, then leave to cool overnight – so that the baklava can absorb all the syrup. If you’re making this for a special occasion, it’s nice to scatter over some torn rose petals (they have to be small pieces) just before serving.