Rub some oil around the inside of a large baking tray.
Put the flour into a bowl, add the xanthan gum and stir to mix the flours together well.
Put the butter and sugar into a mixing bowl and beat together until light and fluffy.
Separate the eggs, add the yolks to the mixing bowl and save the whites for later.
Add the grated lemon rind and 1 tablespoon of lemon juice and beat everything together well.
Sieve the prepared flour to the bowl and stir again.
Mix in the currants.
Cover and chill the dough for 10 minutes.
Pre-heat the oven 180°C, Fan 160°C, 350°F, Gas 4.
Dust your work surface and rolling pin with flour.
Put the dough in the middle and roll it out to the thickness of two £1 coins.
Press a 5cm/2” cookie cutter or cup through the dough to make circles then gently lift them onto the prepared baking tray.
Gather together the offcuts and re-roll to make further biscuits.
Brush the biscuits with the reserved egg white.
Sprinkle the two teaspoons of sugar over the biscuits.
Bake for 18 – 20 minutes until golden.
Transfer cooked biscuits to a wire rack to cool.
When cold store the biscuits a tin.