Activate Pre-ferment (overnight)
Mix the sourdough starter with 100 grams of warm water and 100 grams of bread flour to create the the pre-ferment. The pre-ferment (a.k.a. levain) is an off-shoot of the sourdough starter. Cover and leave on the counter overnight. Because at elevation the environment is cold and dry, keep the pre-ferment in the microwave with a small bowl of boiled water.
Autolyse (60 minutes)
In a large pot, combine 775 grams of bread flour and 100 grams of whole wheat flour with 625 grams of warm water. Mix just until all the flour has been hydrated. Cover.
Let the flours rest at least 60 minutes. This rest is called the autolyse (“auto-lease”), which allows the flours to absorb the water, activating the enzymes which begins the gluten development. This step is critical and cannot be rushed.
Make Dough (10 minutes)
Add the pre-ferment to the dough. Incorporate the pre-ferment by squeezing it through your fingers.
Sprinkle 20 grams of salt over the dough and then add 50 grams of warm water. Poke your fingers into the dough to press some salt deep inside, then fold over itself until the salt and water is incorporated. Cover the bowl again; the bulk fermentation has begun. Because the environment is cold and dry, keep the bowl in the microwave with a small bowl of boiled water.
Bulk Fermentation (~4 hours)
During the first two hours stretch-and-fold the dough every 20 minutes. For each stretch-and-fold, first wet your hand to prevent excessive sticking. Then reach under the dough, grab the bottom and pull up the dough up as far as possible, and then turn it back on top of itself. Rotate the bowl 90 degrees and repeat 7 more times. Recover the bowl. Because the environment is cold and dry, re-boil the water before putting the dough back in the microwave.
If your dough does not seem very elastic, you can repeat the stretch-and-fold process two more times, with a thirty-minute rest in between each. If the dough becomes difficult to stretch and does not want to bounce back, let it rest.
For the final two hours, just let the dough rest. Bulk fermentation is complete when the dough has doubled in size, has a few nice big bubbles on the surface, and a slightly shiny appearance.
Divide and Pre-shape (10 minutes)
Take the dough out of the container onto your unfloured work surface. Sprinkle some flour on top of your dough and divide in two equal halves. Take a half, flip it over and pull each of 4 edges from under onto the top. Flip the folded dough over so the seam is on the unfloured work surface. The work surface grabs the bottom of the dough slightly as you form the boule (ball). Place your fingers on the lower back side of the boule and pull it a few inches towards you. Rotate the boule and repeat until the loose boule is formed with a taught, smooth surface. Be careful to do this process gently so as not to release the gases that have allowed the dough to rise. Repeat with the other half.
Bench Rest (30 minutes)
Cover the boules with tea towels or inverted mixing bowls. Let rest for 30 minutes.
Shape (10 minutes)
Prepare the final proofing containers: either banneton baskets or medium bowls lined with a clean kitchen towel and sprinkled very liberally with flour.
The resting dough should have spread out, but not quite into a pancake shape. If it has formed a pancake you can strengthen it by pre-shaping one more time and waiting another 20-30 minutes.
Repeat the shaping process and place each boule upside down (smooth side down, seam side up) in a proofing container.
Proof (2-12 hours)
If baking immediately, allow the dough to prove at room temperature for two hours, or until it slowly springs back when poked. If time allows, proof in the fridge for 8-12 hours. This allows the gluten to develop further and results in much better flavor.
Final Prep (1 hour)
Place a dutch oven (or pizza stone, if no dutch oven is available) inside the oven and preheat to 450 degrees F for 1 hour. If using a dutch oven, place it on top of a baking sheet to help prevent a burned bottom crust.
If the dough has proved overnight in the fridge, remove and allow to rest at room temperature while the oven is heating. Test the dough. You will know that it is ready to bake when you poke the dough gently with your finger and it springs back slowly. If it springs back very quickly, then it needs a few more minutes. If the indentation stays, then it has over-proved and should be baked immediately.
Turn the dough onto a piece of parchment paper, seam side down. Using a very sharp knife or razor blade, quickly and carefully score the bread in a decorative pattern. The key is to allow the bread a way to expand while baking so it does not split.
Bake (1 hour)
Spritz or sprinkle the dough with water to help create steam for a crisp, crackly crust.
If using a dutch oven, transfer the dough still on its parchment into the dutch oven. Replace the dutch oven lid, return to the oven, and bake for 25 minutes. Then remove the lid, and bake 20-25 minutes more until loaf is deep brown.
If baking on a sheet or pizza stone, bake uncovered for 40 minutes until the crust is deep and dark.
Cool (1 hour)
Cool on a baking rack, avoiding slicing until the loaf is thoroughly cooled, about an hour. The loaf can be left out at room temperature for one day; after that, wrap it in plastic and place inside a zippered plastic bag.
Notes
This recipe contains 1000 grams of flour and 800 grams of water which results in 80% hydration. This assumes that the starter is at 100% hydration.
Given that the amount of fermented flour (flour in the pre-ferment) is 125g, then the inoculation rate is 12.5%. The less the inoculation rate, the more time is needed to bulk ferment and proof the dough, thus the more time for the lactic acid bacteria to produce flavor compounds such as acids. Generally speaking, the higher the inoculation rate, the less tangy the bread will be. Other factors such as adding more whole grain flours to your dough will increase its tang.