Preheat the oven to 160°C/Fan 140°C/gas 3.
Lightly grease a 30cm x 23cm roasting or traybake tin.
Mix together the flour and semolina in a bowl or food processor.
Add the butter and sugar and rub together with your fingertips until the mixture is just beginning to bind together.
Knead lightly until the mixture forms a smooth dough.
Press the dough into the prepared tin and level it with the back of a spatula or a palette knife, making sure the mixture is evenly spread.
Prick all over with a fork, sprinkle over the flaked almonds if using, and chill until firm.
Bake for about 35 minutes or until a very pale golden brown.
Sprinkle with demerara sugar and leave to cool in the tin for a few minutes, then cut into 30 fingers.
Carefully lift the fingers out of the tin with a palette knife and finish cooling on a wire rack.
Store in an airtight tin.
