Focaccia alla Genovese with Natural Yeast

Modified slightly from: http://ilnostroforum.8forum.info/t473-focaccia-alla-genovese-col-lievito-naturale
even if it is in Italian its worth watching the video: http://ilnostroforum.8forum.info/t473-focaccia-alla-genovese-col-lievito-naturale

for the dough:

Sour-dough 150 grams
Water 300 grams
Flour 500 grams
Extra virgin olive oil 20 grams
One teaspoon sugar
Salt 13 grams

Also: extra virgin olive oil and coarse salt

The sour dough has to be refreshed the day before.
In a bowl mix the sour-dough and the water, until the sour-dough has dissolved. Add the olive oil and the sugar.
Add most of the 500 grams of flour and start kneading. Mixing the salt to the remaining flour,'s add the rest and knead for about 10 minutes.
Form a ball with the dough and put it in a plastic or glass bowl,  cover it and let it proof  at room temperature until it doubles in volume. Depending on the season it can take 12 hours or more.
Pour the mixture on a floured surface,  fold it and give it a rectangular shape.
Cover the dough  with a damp cloth  and let it rest for 40 minutes.
Cover the oven's baking tray with a sheet of baking paper, then oil the baking paper very slightly. Drop a good spoon of oilve oil in the middle of the baking tray, put the dough over it and then turn the dough so that it its covered in oil. With your hands flatten the dough until it covers the whole baking tray.
Sprinkle the surface with coarse sea salt; switch on the oven light and add a pan with boiling water to raise the temperature. Put the focaccia in the oven to proof for about an hour.
An hour after sprinkle some olive oil on the focaccia and then press the dough with your fingertips to create the traditional dips. Pour half a glass of warm water over the focaccia. Put it back in the warm oven for another 2 hours.
Remove the focaccia from the oven and preheat it top 250 degrees.
As soon as the focaccia is in the oven lower the temperature to 230 and bake for 15 minutes. Use the grill for a minute or two to get a good colour on the top of the focaccia.