Combine 1,000 grams white-bread flour with 1,000 grams whole-wheat flour. You may not need all of this, but you can use it to continue to feed the starter. Put 100 grams of warm water (about 80 degrees) in a small jar or container and add 100 grams of the mixed flours. Use your fingers to mix until thoroughly combined and the mixture is the consistency of thick batter. Cover with a towel and let sit at room temperature until mixture begins to bubble and puff, 2 to 3 days.
The night before baking, discard all but 1 tablespoon of the mature starter. Mix the remaining starter with 200 grams of warm water and stir with your hand to disperse. Add 200 grams of the white-wheat flour mix and combine well. Cover with a towel and let rest at room temperature for 12 hours or until aerated and puffed in appearance. To test for readiness, drop a tablespoon of leaven into a bowl of room-temperature water; if it floats it’s ready to use. If it doesn’t, allow more time to ferment. This is an essential step. I have had the leaven get ready much quicker when it is especially warm. Alternatively, mixing in the early am with warm water and a warm environment can lead to a dough that can be baked same day.
In a large bowl, combine 200 grams of leaven with 700 grams of warm water and stir to disperse. (Reserve remaining leaven for future loaves--it is now your starter and will require regular discarding and feeding the same as above. If you wish to slow down the need to feed it daily, store in the refrigerator.)
Add 20 grams fine sea salt and 50 grams warm water to the dough, and use hands to integrate salt and water into dough thoroughly. I find this is a fun little exercise squeezing the dough through your hands until it comes apart and then back together again.
Cover dough with a towel and transfer to a warmish location, 75 to 80 degrees ideally. Let the dough rise for about 30 minutes. Fold dough by dipping hand in water, taking hold of the underside of the dough and stretching it up over the rest of the dough. Repeat this action 3 more times, rotating bowl a quarter turn for each fold. Do this every half-hour for 2 1/2 hours more (3 hours total). The dough should be lighter feeling and increase in volume 20 to 30 percent. If not, continue to let rise and fold for up to an hour more. A small note here...I have done this first bulk fermentation rise overnight, where I can assure you I was NOT waking up every half hour to turn and mix the dough. It was also cooler than 75 degrees, and it took a few more hours to get to what I would consider lighter. You have some ability to personalize your dough here, based on the warmth of the room, and the length of time you want this step to take.
Transfer dough to a lightly floured work surface and cut in two. Try not to allow too much additional flour to work into the dough, but by turning the dough, work it into taut rounds* (link to video on how to do this below!) Place the dough rounds on a work surface, cover with a towel, and let rest 30 minutes.
Line two 10- to 12-inch bread-proofing baskets or mixing bowls with towels. Use some of the flour (this is where you would use some rice flour if you want a lovely bright white outside, or you can do a mix of whole wheat and rice flour) to generously flour towels.
Transfer rounds, seam-side up, to prepared baskets. Cover with a towel and return dough to the 75- to 80-degree environment for 2 to 3 hours.
About 30 minutes before baking, place a Dutch oven or lidded cast-iron pot in the oven and heat it to 500 degrees. Very carefully remove the heated pot from oven and gently turn 1 loaf into pan seam-side down. Use a lame or a sharp knife or kitchen scissors to score the top of the bread a few times to allow for expansion, cover and transfer to oven. Reduce temperature to 450 degrees and cook for 20 minutes. Carefully remove lid (steam may release) and cook for 20 more minutes or until crust is a rich, golden brown color. I like to take the bread's temperature aiming at about 210 degrees so I know it is fully cooked.
Transfer bread to a wire rack to cool for at least 25 minutes before slicing. The bottom of the loaf should sound hollow when tapped. Increase oven temperature to 500 degrees, clean out the pot and repeat this process with the second loaf.