2Combine flour, sourdough starter discard, olive oil, rosemary, and kosher salt in a bowl. Mix with a wooden spoon to incorporate. Turn out onto a work surface and knead until a smooth dough forms.
3Divide the dough into 2 pieces and shape each into a small rectangle. Lightly flour a large sheet of parchment paper and place 1 piece of dough on top. Lightly flour the top, roll into a large 1/16-inch thick rectangle, and cut into 1¼-inch squares. Carefully transfer the parchment paper and dough to a baking sheet. Repeat with remaining dough.
4Spray tops with cooking spray and sprinkle with flaked sea salt.
5Bake until crackers are golden brown, about 24 minutes, turning halfway through.
6Remove from the oven and let cool on a baking rack.
Notes
Don't discard sourdough starter — this recipe is an excellent use for the discard every time you feed your starter. Fully cooled crackers can be stored in an airtight container at room temperature for up to 1 week.