• Make sure your sourdough starter is very bubbly and active before mixing the dough. Feed it 4-6 hours before you need it, and wait for it to rise in the jar, at least doubling in height, before using.

Cook the onions

• Warm up a large nonstick pan on medium heat with the butter or oil and then, add the chopped onions, fresh thyme, salt and ground black pepper.
• Stir often and cook on medium-low heat until soft and caramelized to your preference, about 15-20 minutes (or even longer). Once done, remove from the heat and set aside to cool down. The onions can be prepared ahead of time.

Mix the dough

• In a large mixing bowl, add the water and sourdough starter, then mix until combined. Then, add the bread flour, whole wheat flour and salt. Keep mixing until combined, which can take a few minutes. I like to start with a dough mixer, then use my hands. The dough will be shaggy at this point.
• Once done mixing, cover well and rest for 20-30 minutes.

Stretch and fold (+ incorporate the onions)

• Perform 3 series of stretches and folds with 30 minutes of resting time in between. During the first session, you will also incorporate the cooked onion.

  Stretch and fold method: transfer the onion over the dough, then grab the dough on the opposite side of the bowl, pull it up until it cannot stretch anymore, then fold it back down on the dough. Rotate the bowl about 90º and repeat with the dough on the opposite side. Keep going until the dough is too tight to stretch (I usually do about 4 stretches per session) and the onion is roughly incorporated (they will keep mixing in the dough in the next series). Cover, rest for 30 min and repeat 2 more times.

Bulk fermentation

• Once done with the stretches and folds, then cover well and place in a cozy spot for the dough to ferment and rise. How long the dough needs to ferment will depend on the dough temperature and the room temperature. The warmer, the faster the fermentation will go. In my case, I fermented my dough for about 5-6 hours (my dough was around 24ºC or 77-78ºF), but yours might need more or less time. The dough is done fermenting once it looks lighter, puffy, jiggles when moving the bowl and you might see some large bubbles on top of the dough. It will also have risen in the bowl, although it won't necessarily double in size.

Pre-shape

• When the fermentation is done, transfer the dough to a clean and very lightly floured surface. Then, grab the edges of the dough, pull slightly and bring them back in the middle to roughly shape a ball. Flip the ball so the seam side is on the counter, then cover and rest for 30 minutes.

Shape

• Flip the dough so the smooth side is back on the counter. Stretch the dough to shape a square, fold the top part to about the middle of the square, then fold both sides as well. Grab the top edge and roll towards the other edge until the seam side is on the counter while tucking the dough into a ball.
• To tighten the surface dough, then make sure there is barely any flour on the counter, use both your hands to grab the top part of the dough and drag it on the counter towards you. Rotate slightly and repeat. Do this until the dough feels tight on top and forms a nice ball (make sure to check the video down below).

Final proof (cold proofing)

• Add flour generously to your proofing basket and a little over your dough. Then, flip the dough into your basket with the seam side up. Cover well and place in the fridge for the final proof (5-36 hours).

Bake

• About 30-60 minutes before you want to bake the bread, transfer the Dutch oven into your oven and preheat to 450ºF.
• When ready to bake, flip the dough on a large piece of parchment paper, score as you wish and transfer to your preheated Dutch oven. Cover and bake for 30 minutes. Then, turn the heat down to 410ºF, remove the cover and bake for 5-7 minutes or until the crust turns golden brown.

Cool down

• Once done, remove from the Dutch oven and let the bread cool down completely before slicing and enjoying (at least 2 hours).