To make the overnight starter, in the evening stir down the 100% hydration sourdough starter, and remove 22 g in a bowl. First add the water and stir well. Add flour and stir. Cover with plastic wrap, and let it rest at room temperature for 12-14 hrs (overnight). (For the 100% hydration sourdough starter – you either make it to have it your own - as shown here – or take from a friend who has or buy from a specialized store).
In the morning, in a large bowl mix the flours with water and the overnight starter until well combined. Let the dough rest (autolyse) for 30 minutes.
Add the salt and knead by hand for 5-8 minutes.
Wipe the inside of a wide bowl with a little vegetable oil. Place the dough into this wide bowl so the dough can be stretched and folded without removing it from the bowl. Cover with plastic wrap.
Let the dough ferment at room temperature for 2.5 hours with folds at 50 minutes. The folding is shown here.
Turn the dough out onto a floured surface. Divide it in two pieces. Shape them in form of a ball. Sprinkle the balls with flour, cover and let rest for 15 minutes.
Meanwhile prepare two round bowls with 2 kitchen towels, sprinkled with flour. Add the dough balls seam-side-up, cover with the edges of the towel and proof for 2 or 2.5 hours at room temperature. You will know it's done when pressing the dough with the finger the it comes slowly back.
Preheat the oven to 475 F (240 C) with baking stone or in case you don't have one preheat a baking sheet. Place a small pot with water at the base of the oven to create steam.
Turn the proofed loaves onto a parchment and score them. You can see some tips here. Carefully transfer it to the preheated baking stone or back of the baking sheet.
Bake for 15 minutes with steam then remove the pot with water from the oven and bake for another 15- 20 minutes without steam.
Cool on a wire rack. Let them cool completely before cutting for at least 2 hrs.