Make the Sourdough Crust: Mix flour, sugar, and salt. Add cold butter cubes, coating with flour, then pinch and flatten until you have a crumbly texture. Keep mixture cold.
Add sourdough starter and stir just until combined. Dump onto parchment, shape into a disc, wrap, and refrigerate for 30 minutes or longer.
Prepare the Filling: In a bowl, combine blueberries, maple syrup (or sugar), lemon juice, lemon zest, vanilla, and salt. Let sit for 10–15 minutes. Add the cardamom and cornstarch and mix gently to coat.
Assemble the Galette: Preheat the oven to 375°F (190°C). Roll dough on floured parchment into a 12-inch circle. Transfer to a baking sheet.
Use a slotted spoon to add blueberry filling to the center, leaving a 2-inch edge. Discard liquid. Fold dough edges over filling. Brush the crust with egg and sprinkle with sugar.
Bake: Bake for 35–40 minutes until golden and bubbly. Cool slightly before serving to help set the juices.
Serve: Dust with powdered sugar or lemon zest curls. Pair with whipped cream or ice cream.
Notes
Frozen blueberries can be used but thaw and rinse them first. They may also require extra cornstarch.The galette may bubble over slightly, so use a rimmed baking sheet for easy cleanup.The dough will be softer than a traditional pie crust because of the sourdough discard. Chilling thoroughly is important for easy handling and a flaky finished crust.Don’t skip chilling the crust, as it ensures flakiness. If it gets too sticky while rolling it out, just toss it in the freezer for a few minutes. Or, place another piece of parchment paper on top to help roll it out.When folding over the edge, it’s a bit easier if you lift up the parchment and fold the crust over the filling using the parchment to do the work. Then just peel the parchment away from the dough.Let the galette rest before slicing to help set the juices.How to Store
Store leftover galette covered in the refrigerator for up to 3 days.
Reheat slices in a 350°F oven for 8–10 minutes to crisp the crust again.