Heat the oven to 180C/160 fan/gas 4. Place the butter and sugar in a bowl and beat together with a wooden spoon until well combined. Add the egg yolk, lemon zest and nutmeg and beat again.
Add the flour, salt and currants and mix everything together to make a firm dough, using your hands if necessary. Form into a puck-shape, wrap and chill in the fridge for 30 mins.
Line two large baking sheets with baking parchment and lightly dust your work surface with flour. Cut the dough in half and roll out to a ½ cm thickness. Cut out biscuits using a 6cm fluted cutter, lift onto one of the baking sheets with a palette knife, leaving a little space in between. Repeat with the remaining pastry to make a second tray of biscuits, re-rolling the off-cuts. Chill for 30 mins.
Bake for 7 mins, then remove from the oven, brush with egg white, sprinkle with extra sugar and return to the oven for 7-8 mins or until lightly golden brown. Leave to cool on the trays for 5 mins then carefully transfer to a wire rack to cool completely.