1. In a food processor, combine 250g (2 3/4 cups) of gluten-free flour mix and 1/2 teaspoon of salt with 115g (1/2 cup) of cold butter. Process the mixture until it resembles coarse crumbs.

2. Gradually add 60ml (1/4 cup) of cold water, 60g (1/4 cup) of sour cream, and 1 beaten egg, if desired. Process the mixture until a dough forms.

3. Knead the dough quickly until it comes together. Do not overwork.

4. Wrap the dough tightly in plastic wrap and refrigerate for at least 30 minutes to allow it to firm up.

5. Preheat the oven to 200°C (400°F).

6. Roll out the dough on a lightly floured surface to a thickness of about 3mm (1/8 inch).

7. Use the dough to line a tart pan or shape it into your desired forms.

8. Prick the bottom of the pastry with a fork to prevent it from bubbling up during baking.

9. Bake the pastry for 5-10 minutes, or until it's lightly golden.

10. Allow the pastry to cool before filling and serving.