Preheat oven to 160 degrees C. Grease a 20x30cm pan.
To make crust: In a large mixing bowl beat together the butter, 2 cups of the flour and icing sugar until fluffy. Scrape mixture into the pan and smooth. Bake 15 minutes.
In a medium bowl beat the eggs and sugar. Grate the lemon and add the zest, along with 6 tablespoons of lemon juice, to the egg mixture. Sprinkle 1 tablespoon of flour and the baking powder over the mixture. Fold in the nuts and mix until well blended. Pour the lemon mixture into the crust.
Bake for another 35 minutes until filling is set. Let cool before cutting.