Add all the ingredients (apart from the Demerara Sugar) into a large food processor and whizz until a soft ball of dough forms.
Transfer the dough to the prepared tin and press it out until the surface is even. Working your way around the outside edge, press the dough using your thumb to imprint a scallop pattern (or use the prongs of a fork). Use a knife to mark out 8-10 wedges, (depending on how large you would like each serving) but don't cut all the way through.
Chill the shortbread in the fridge for about 30 minutes.
Preheat the oven to 150ºC/130ºC fan/gas mark 2. Using a fork, prick the shortbread all over the surface and sprinkle evenly with Demerara Sugar. Bake for about 1 hour, or until lightly golden.
Leave the shortbread to cool a little before cutting into the marked out wedges.