Combine flour, salt and yeast in a medium mixing bowl. Make a well at the center and pour in water slowly. Bubbles should be forming where yeast is in contact with water. Mix everything until it becomes a shaggy dough. Cover bowl with aplastic lid or cling film and allow to proof for 18 to 20 hours at room temperature, that is, if your room temperature is at least 21 degrees Celsius consistently. Otherwise, cold proof inside the refrigerator. Do not overproof (past 24 hours, or at room temperature without air conditioning), otherwise the dough won’t hold its shape once baking.
The next day, take out bowl and remove lid or film. Check glutens in the dough; these are protein strands that look like cobwebs, when flour is mixed with water and a leavener. It traps gas bubbles during fermentation to allow dough to rise and give it texture. Sprinkle some more flour on dough and shape into a ball by lifting the end and pulling towards the center. Do this clockwise to full turns, for 2 minutes. Allow dough to rest for about 15 minutes. Repeat the process twice until dough becomes smooth. Allow to rest for an hour.
Tear a sheet of parchment paper and sprinkle some flour at the center. Cut 4 long strands of twine, about 22 inches each, and lay them on top of the paper, crisscrossed, shaped like an equal pie chart. Carefully invert bowl to the center of paper where the twines intersect, and tap the bottom of the bowl to loosen the dough. Scoop out any remaining dough that stuck in the bowl, and smoothen it in the exposed dough. Shape the dough to a tight round ball and sprinkle more flour on top to smoothen the edges.
Heat the oven to 230 degrees Celsius with a covered Dutch oven inside, or a heavy cast iron deep skillet with lid. While oven is heating, tie the twine from end to end towards the center of dough. Repeat until all the twine has been knotted. Using a small blade, score dough by cutting diagonal strips and lines in each panel at varying lengths, to form a leaf pattern. Allow dough to rest for another hour so it can expand further.
Carefully remove Dutch oven or heavy skillet using heavy mitts, and transfer to countertop. Lay a folded towel on top to protect the surface. Remove the lid. Lift the parchment paper with both hands and carefully place the dough at the center of the Dutch oven or skillet. Cover with lid.
Bake for 45minutes. At the 30 to 35 minute mark, remove Dutch oven or skillet, transfer to the countertop, and remove the lid. Return the vessel to the oven and bake uncovered, for the remainder of the timer, until surface has a deep golden browned. Allow to cool before cutting the twine and slicing.