Mix together all the ingredients for the pre-ferment the night before in a small bowl. Loosely cover and leave on the counter overnight, or at least for about 6 to 8 hours. You can also do this first thing in the morning, if you’re baking the Naan Sangak for dinner.
The next morning, mix together all the ingredients for the dough except the water in a large bowl. Slowly add as much water as necessary and knead till you have very slack and wett-ish dough. The dough should be slack. You will need to wet your hands to handle the dough without sticking.
Once it has come together, wet your hands and fold dough on itself, pushing your hand under the dough and pulling up from one side towards the other. Turn the bowl and repeat until you have done this 4 times, rotating the bowl through one full circle. The dough should be reasonably smooth now. Repeat this folding once again after about an hour.
Cover the bowl loosely and leave the dough to rise till almost three times it volume. This will take between 2 to 3 hours.
About half an hour before you are ready to bake the Naan Sangak, pre-heat your oven. Set the oven at 330C (450F). Put the baking tray with the pebbles on the middle shelf of the oven so the pebbles get really hot.
While pebbles are heating, start shaping the dough. Using your palms slather water generously on the back of another large cookie tray or pizza peel. Turn the risen dough out onto the wet tray. It will very slack which is what we want. Wet your hands and gently guide and flatten the dough into a rough rectangle. Make sure that one end of the rectangle is very close to one short side of the tray. Gently lift the bread up and down again to make sure it is not stuck to the pan.
When the stones are really hot. VERY, VERY CAREFULLY take the hot tray out of the oven. DO NOT TOUCH THE STONES WITH YOUR HANDS! Work quickly. Don’t forget to spray the stones generously with oil so they’re well coated.
Start at the short edge of the pebble tray that’s away from you. Tip the edge of the tray with the dough at the edge of the pebble tray that’s away from you. The dough should begin to slide off the back of the wet tray. If it does not, gently nudge any part that is sticking with a thumb or finger.
Gently pull dough tray back in a smooth slow motion as the rest of the dough slides and lands on the pebbles. This will stretch the dough out into a thinner rectangle. Let all of the dough slide off onto the pebbles. Help it along the way but make sure you don't touch the hot pebbles! Once the dough is on the stones, it WILL stick for the first part of baking. Do not try to rearrange the dough.
It takes about 10 minutes or so to bake the bread. Turn the tray around from time to time to ensure even baking. Remember to USE THOSE OVEN MITTS! To check to see if the bread is done, use blunt-nosed tongs to gently lift the bread from the pebbles. The bread will not come off the pebbles until it is cooked so it might need a little longer in the oven. Do not over bake it.
Some of the pebbles may still stick to the bread even after it is cooked. That’s alright. They will come off once the bread is completely cool. Don’t forget the pebbles will retain heat for a long time. You can turn off the oven once the bread is done and leave the tray of pebbles in it to cool slowly.