This is an overnight process, so give yourself plenty of time for this recipe.
In a medium sized mixing bowl, combine the sourdough starter, flours, sugar and water using a spatula or a fork. You will need to adjust the water according to how hydrated your sourdough starter is: you should be aiming for a sticky dough that's a little damper than typical bread dough.
This is a no-knead dough, so all you have to do is mix the ingredients for about a minute until they come together. Easy!
Cover and let sit at room temperature for 1 hour.
Add the salt, additional water and oil, incorporating the added ingredients into the dough without actually kneading it. You might have to slightly stretch the dough and fold it onto itself several times.
Cover and allow to rest at room temperature for 30 minutes.
Once again, fold the dough a couple of times, cover and allow to rest for 30 minutes.
One last time, fold the dough a couple of times, cover and allow to rest for 30 minutes.
After the final 30 minutes of resting, fold the dough, cover, and place in the fridge overnight.
When you are ready to bake the bread, remove the dough from the fridge. Cut the dough in half, keep covered in the same container and allow to sit at room temperature for 1 hour.
Fill a baking sheet with pebbles (see notes below) and place on the lower rack of the oven. Preheat the oven to 525°F.
You will need a small bowl of water, and a large flat plate that can be made wet.
First wet your hands and then wet the surface of the plate. Make sure you don't get the plate too wet, however.
Place the first half of the dough on the wet plate and do your best to spread it out into a rough oval shape, about 8 inches long. Practice makes perfect but don't fuss over this too much. Each bread will have its own unique shape!
Carefully remove the HEAVY tray full of HOT pebbles from the oven and get ready to transfer the dough onto the tray.
Slowly tip the plate so that the dough begins sliding off of the wet plate onto the pebbles. Gently pull the plate back in a smooth slow motion as the rest of the dough slides and lands on the pebbles. Help it along the way but make sure you don't touch the hot pebbles! Once the dough is on the pebbles do not attempt to rearrange it, as it will already be stuck to the stones.
Sprinkle the seeds over the flat bread.
Carefully place the tray back in the oven, and bake for 8-10 minutes depending on how the dough was spread out on the pebbles. The bread should be mottled brown as shown in the picture.
Once baked, carefully remove the tray from the oven, and using tongs, remove the bread carefully, including any pebbles that are stuck to the bread. No way around this, so don't worry about it. Place on a cutting board until you can remove the pebbles without burning yourself. Double check to make sure no pebbles are embedded into the bread.
Repeat with the second half of the dough.
This bread is best served fresh and hot out of the oven, of course. If you need to store the bread, keep it in an airtight container and reheat it in the oven or toaster oven before serving.