In a bowl, combine sourdough starter with flours, olive oil, herbs and salt. Mix to combine, kneading until the dough comes together in a smooth ball.Wrap tightly in plastic wrap and refrigerate for at least 30 minutes or up to 24 hours.Position oven racks in the upper 1/3 and lower 1/3 of oven; preheat to 350 degrees F. Line two baking sheets with parchment or silicone baking mats.Cut dough in half; put one half back in the fridge while you roll out the other. Cut dough again into 4 smaller pieces.Roll out each piece into an oblong rectangle. You can do this with a rolling pin on a lightly floured surface, or using a pasta roller for super thin crackers. I like to roll my dough out to the #6 thickness setting (out of 8). If you are rolling by hand, just roll it as thin as you possibly can.Lay out two oblongs of dough side by side (not overlapping) on each baking sheet.Spritz or brush lightly with water; sprinkle with flake salt.Bake for 12 to 15 minutes or until lightly golden brown and crispy, rotating the pans top to bottom and back to front part way through baking.Let cool, then transfer crackers to a cooling rack. Repeat with remaining dough.Crackers will keep in an airtight container at room temperature for up to one week.