Sift the flour into a bowl along with the salt and set it aside.
In a separate bowl, cream together the butter and sugar with an electric mixer until smooth and creamy.
Add the lemon thyme to the sugar-butter mixture and mix on low speed.
Gradually add the flour to the mixture, stirring until dough comes together.
Shape the dough into a flattened ball, wrap in clingfilm, and chill in the fridge for at least 20 minutes.
Preheat oven to 180C, grease a baking tray, and press the dough onto the tray. Score the surface and prick with a fork.
Bake for 15-20 minutes until pale golden, then cool before transferring to a wire rack.
Sprinkle with caster sugar and store in an airtight container before serving.