In a clean bowl or food safe plastic container mix all the flour + 500ml of water until absorbed. Add the levain, salt and extra virgin olive oil and keep mixing. You should mix with the “stretch and fold” technique: wet your fingers, slide your mixing hand under the dough, pull up a flap of dough and fold it over at the center. As you do this, rotate the bowl: you should fold 4 times per round, and keep folding until the dough starts developing tension. Make sure you keep wetting your fingers as you fold.
Add remaining water very slowly, little by little (using the “bassinage” technique: pour a little water, let it pool on the surface of dough, press down gently to create small dimples in the dough, let it rest for a few minutes until most of the water on the surface is absorbed; then fold a few times; then repeat with another small amount of water). Stop if dough starts ripping, even if you haven’t added all the remaining water.
Cover the container with a lid, a cotton towel or some Saran wrap and bulk the dough at room temperature for 2.5 hours with folds every 30 minutes (for folds: I do one regular fold, at the beginning, then coil folds; please refer to YouTube for video tutorials on regular folds and coil folds).
Cover the container and put in the fridge overnight (for no less than 17 hours).
Prepare two baking sheets lined with a cotton or linen towel (like a baguette “couche”). The towel should be double the length of the baking sheet: you will have to pinch it up as you place your dough rectangles on it, so as to create a set of parallel grooves, for the ciabattas to rest in. Alternatively, you can use two overlapping towels. It is of the outmost importance that you flour the towel abundantly or the dough will stick to it and not release properly when it’s time to bake.
Take the dough out of the fridge. Dump it gently on a lightly floured surface, helping it detach from the container with a plastic scraper. Flour the surface of the dough all over and with the help of a metal bench scraper, cut it in 15cm long rectangles (these will be your ciabattas). Start from the edge of the dough that’s closer to you, go in with the scraper perpendicularly to the dough, repeat on the two short sides of the rectangle and pull it away. Gently stretch it width-wise with your fingers and transfer to the couche. Repeat with the rest of the dough. Depending on the size of your sheet, you should be able to fit 1 or 2 ciabattas per row.
Cover the baking sheets and proof at room temperature for 2 hours.
Put a baking stone (alternatively you can use a heavy duty, thick baking sheet, though it’s not ideal) on the central rack of your oven, and place a baking mold or Pyrex bowl on the bottom of the oven, bringing the temperature to 464F. When it’s hot, take the stone out and sprinkle it with semolina flour.
Prepare a small wooden board (or a cutting board), roughly the length of a proofed ciabatta and not too thick, dust it with semolina or rice flour. Working with both hands, bring the board close to the edge of your couche, near the first ciabatta, and lifting the couche flip the ciabatta over on the board, so that the bottom of the ciabatta becomes the top.
Transfer the ciabattas to the baking stone, leaving enough space between them. When it’s fully loaded, put the stone back in the oven, carefully pour some water in the container on the bottom (this will immediately create steam, which helps the ciabattas rise), close the oven door and bake. After 15 minutes carefully take the container with the water out, lower the temperature to 428F and keep baking for additional 15 minutes – or until the ciabattas look golden and sound hollow if you tap them on the bottom. Put the baked ciabattas on a cooling rack and proceed with the rest of the dough.