In the morning, check your levain. It should have bubbles on the surface- this means it’s ready to go. Tip in 600g tepid water and stir with your spatula to dissolve the levain. Add 1 kg strong white bread flour and stir well to create a shaggy dough. Cover and leave for 30 minutes.
After 30 minutes, sprinkle over the salt and tip over 50g water. Use your hands to pinch and twist it into the dough, until it comes together as a smooth, slightly sticky dough. Cover and leave in a warm place for 30 minutes.
Uncover the dough, then grab one side of the dough, stretch it out and pull it over itself. Turn the dough clockwise and repeat four times. Cover and leave in a warm place for 30 minutes.
Repeat the stretching and pulling three more times (four times in total), leaving to rest for 30 minutes each time. When you have done this, cover the dough and leave it in a warm place for 3 hours.
Tip the dough onto a floured surface. Split into two, and form each one into a ball. Leave uncovered for 30 minutes.
Prepare two proving baskets by flouring them heavily. Take one of the dough balls and form a tight ball by pulling the sides out and folding them in on itself. Place it in one of the floured baskets. Repeat with the other dough ball. Put the baskets in the fridge overnight.