Grease the paper lining the bottom of the tin to help it stay in place.
Preheat the oven to 170°C/150°C fan/gas mark 4. Grind your pre-toasted pistachio nuts in a food processor until you have a fine mixture.
Cream together the butter and sugar until pale and fluffy, then add the RSPCA Assured eggs and the flour - one tablespoon at a time - mixing constantly.
Make sure your mixture is smooth and consistent, then place half into another bowl.
In your first bowl, add most of the ground pistachios (keeping a tablespoon back for decoration) and a few drops of the green food colouring. Mix until well blended. If the mixture is a little stiff fold in some RSPCA Assured milk.
In your second bowl, add the rose water, lemon zest, 2 tsp lemon juice and a few drops