You will need one large baking sheet lined with baking parchment.
Method
Preheat oven to 150C Fan / 170C / 325F / GM3.
Cream butter, lemon zest and icing sugar until pale and fluffy.
Gently mix in flour, semolina and finally hazelnuts.
Bring together, form into a flat disc and then roll out between two sheets of baking parchment to around 5-7mm thick. Place on the baking sheet and put in the fridge to rest for 30 mins or so.
Cut into 5cm squares, reuse trimmings, place squares on the baking sheet 1cm apart and bake for 20-25 mins until pale gold.
Remove from oven and immediately dust with caster sugar.
Cool on a wire rack. Best eaten within 2-3 days or can be frozen.