1. Whisk together flour and salt in a mixing bowl

2. Cut cold butter into the flour mixture until coarse crumbs form

3. Add sourdough starter and mix, adding cold water as needed until dough comes together

4. Shape dough into a ball, wrap and chill in the refrigerator for at least 30 minutes

5. Roll chilled dough into a 12-inch circle on parchment paper

6. Mix blackberries, sugar, cornstarch, lemon juice, and vanilla extract; spoon onto dough leaving a 2-inch border

7. Fold dough edges over the filling, overlapping folds to form a rustic crust

8. Brush crust with beaten egg and sprinkle with coarse sugar

9. Transfer galette on parchment to a baking sheet and bake at 400°F (200°C) for 35 to 40 minutes until golden and bubbling

10. Let galette cool for 15 minutes before slicing and serving with whipped cream or ice cream