In a jar, add 1 tablespoon of flour and 1 tablespoon of water. Mix well and cover loosely with a lid. Feed twice a day for the first 3 days. On days 4, 5 and 6, discard half of the starter before adding 50 grams of flour and 50 grams of water once per day. After each feed, place a rubber band to mark where the dough is so that you can measure the activity of the starter. During these days you should see bubbles form and the dough rise and fall. These are good signs of a health active starter. By day 7, it should be ready for use (after a good rise). When you are done with your starter, put it in the fridge. The starter should last indefinitely, as long as you feed it once a week or so. When you are ready to make bread again, bring it to room temperature and feed it to make it active before using.
In a jar, use a digital scale to measure out the bread flour, warm water and active sourdough starter and mix well with a spoon. Cover it loosely with a lid and place a rubber band to gauge the rise. If you do not get a good rise, unfortunately, this means that the starter used was not active enough and you will need to start over. Ideally, the levain should be made the night before you make sourdough bread.
In a large bowl, use a digital scale to measure out the bread flour, whole wheat flour, and warm water. Add the previously made levain and combine the ingredients with a spatula (and hands) to form the dough. Cover the dough with a damp towel or plastic wrap and allow it to rest in a warm environment for 30 minutes.
In a ramekin, combine the kosher salt and water. After the dough has rested for 30 minutes, dimple the dough and pour the salt/water mixture onto it before performing the first set of "stretch and folds". Make sure your hand is wet to avoid the dough sticking to your fingers. Pick up a side of the dough, stretch it and fold it over to the center. Then rotate the bowl slightly, lift another side and repeat. Keep going until all sides of the dough have been stretched and folded. Repeat the stretch and fold process 3 more times with 30 minutes of rest in between.
After the 4th stretch and fold, allow the dough to bulk ferment in the warm environment for 2 hours.
After 2 hours, you are ready to shape the dough. Turn the dough out onto a floured surface. De-gas it by gently pressing on it and shape the dough by using both hands to roll the dough in a circular motion. The friction between the surface and the rolling motion will shape the dough into a smooth ball. Place it onto a well dusted proofing basket. Cover it up with plastic and let it cold ferment in the fridge for 6 to 12 hours.
Preheat your oven to 500°F along with your dutch oven, so that it is nice and hot and ready for you. When ready, take your dough out of the fridge, place a piece of parchment paper over the top of the basket and invert the dough onto it. Score the dough with a bread lame. At this point, take the hot dutch oven out of the oven and carefully lift the ends of the parchment paper to transport the dough into the dutch oven. Spray the dough with a few sprays of water which will help promote a nice crust on the bread. Carefully place the lid on the dutch oven and place it in the oven to bake for 20 minutes.
After 20 minutes, remove the lid and bake for another 10 minutes or until brown. Allow the bread to cool for an hour before you cut into it. Enjoy!