In a large bowl, mix the flour, fine salt, sourdough starter, melted butter and herbs to make a smooth dough. It shouldn't be too sticky: if it is, add a teaspoon of flour.
Divide the dough in half and shape each half into a small rectangular slab or put in oblong tupperware boxes and cover with cling film. Refrigerate for at least 30 minutes, until the dough is firm, but don't leave it longer than 2 hours or your crackers will be very sour.
Preheat the oven to 180 C. Whilst it preheats, lay down 2 pieces of baking parchment that are each roughly the size of your baking sheets. Lightly flour your rolling pin, and the top of the dough.
Working with one piece at a time, roll the dough out onto your baking parchment as thinly as possible (1mm thin, if you can). The dough may have uneven edges but try to make it as even as possible.
Transfer the dough and parchment together onto a baking sheet. Lightly brush with oil and sprinkle your coarse salt over the top of the crackers. Cut the dough into squares with a sharp knife or a pizza cutter.
Bake the crackers for 15-20 minutes, until crackers are starting to brown around the edges. You may want to reverse the baking sheets after 10 minutes: both top to bottom, and front to back - for a more even bake.
Remove the crackers from the oven and leave to cool. Store airtight at room temperature for up to a week.
Pack them into sealable bags or small boxes for a great snack, or serve with cheese and chutney.