Preheat your oven to 300°F (150°C). Gather your ingredients and equipment.

Combine flour, icing sugar, salt, grapefruit zest, and thyme in a food processor. Pulse until combined. Add cubed butter and pulse until a rough dough forms.

Transfer the dough into the tart pan, pressing it evenly across the bottom and up the sides.

Bake in the preheated oven for 35-40 minutes, until golden brown on the edges.

Let the shortbread cool in the pan for about 10 minutes, then slice into 16 pieces. Cool completely on a wire rack.

Whisk together grapefruit juice and icing sugar until smooth. Adjust icing consistency if necessary.

Dip the tops of the cooled cookies into the icing. Let dry on a wire rack, garnishing if desired.