Preheat your oven to 300°F (150°C).

In your food processor, combine the unbleached all-purpose flour, icing sugar, salt, grapefruit zest, and fresh thyme. Pulse the mixture until everything is blended well, about 10-15 seconds.

Cut the cold vegan butter into small cubes and add it to the dry mixture in your food processor. Pulse until the mixture resembles coarse crumbs, which should take about 20-30 seconds.

Gather the crumbly dough and press it firmly into the bottom and up the sides of your fluted tart pan.

Transfer the tart pan to your preheated oven and bake for 35-40 minutes.

Remove the pan from the oven and allow it to cool for about 10 minutes. While still warm, cut the shortbread into 16 pieces.

For the icing, whisk together the sifted icing sugar with freshly squeezed grapefruit juice until you reach your desired consistency.

Dip the tops of the cookies into the icing, allowing the excess to drip off. Set them on a wire rack to let the icing dry.

Before the icing fully dries, garnish each cookie with a sprinkle of fresh thyme or pearl sugar.