Mix together the flour, salt, sourdough starter, butter, and herbs to make a smooth (not sticky), cohesive dough., Divide the dough in half, and shape each half into a small rectangular slab. Cover with plastic wrap, and refrigerate for 30 minutes, or up to a couple of hours, until the dough is firm., Preheat the oven to 350°F., Very lightly flour a piece of parchment, your rolling pin, and the top of the dough., Working with one piece at a time, roll the dough to about 1/16" thick. The dough will have ragged, uneven edges; that's OK. Just try to make it as even as possible., Transfer the dough and parchment together onto a baking sheet. Lightly brush with oil and then sprinkle the salt over the top of the crackers., Cut the dough into 1 1/4" squares; a rolling pizza wheel works well here., Prick each square with the tines of a fork., Bake the crackers for 20 to 25 minutes, until the squares are starting to brown around the edges. Midway through, reverse the baking sheets: both top to bottom, and front to back; this will help the crackers brown evenly., When fully browned, remove the crackers from the oven, and transfer them to a cooling rack. Store airtight at room temperature for up to a week; freeze for longer storage.