Mix the flour with the salt, then start adding the water. Add the sourdough (detail).
Always mixing, add the water and oil, until you obtain a smooth dough. (It usually takes about 15 minutes).
Let the dough leaven in a bowl, covered with a transparent film, at ambient temperature. It should double its size in 3 to 4 hours. Otherwise (my best choice), let is leaven in the fridge, for about 18-20 hours.
Remove the dough from the bowl and gently put it on a kitchen top (or a chopping board) with some flour.
Use your fingertips to shape the dough and let some of the air leave the dough. Put the dough into an oiled baking tin.
After a second leavening of about two hours, make dimples in the dough with your fingertips, add the extra-virgin olive oil, the salt and the rosemary on top.
Turn on the oven at 200 °C (i.e. 392 °F) and after a brief rest of 15 minutes, put the tin into the oven.
After 20-25 minutes your Italian focaccia is ready. Buon appetito :)