This dough had a 8.5-hour first rise (calculated from the initial mixing) at 74°F and a 12-hour refrigerated final proof. You can modify this timing as needed with different ambient/dough temperatures.

In the bowl of a stand mixer, add the flours, water, sourdough starter, and salt. Mix on low speed with the dough hook attachment for 1-2 minutes. The dough mixing and re-mixing below can be done by hand if you don't have a stand mixer, or you can mix by hand and do several rounds of stretching and folding with rests in between over the course of a couple of hours.

Cover the bowl and let the dough rest for one hour.

Mix again, now on med-high speed, for 8-12 minutes. The dough will wrap around the hook and look silky by the end of the mixing.

Transfer the dough to a straight-walled container if you have one. Cover and let it rise until it is between 1.5x and 2x the original size, an expansion of 50-100%.

Transfer the dough to a lightly floured work surface and shape it to your preference (boule, batard, oblong loaf). Make sure you build tension on the surface of the dough and dust it with flour before placing it seam-side up in your proofing basket.

Cover the basket and refrigerate it for 12+ hours.

Preheat your oven and baking vessel to 500°F for 30 minutes.

Directly from the refrigerator, flip the dough out of the proofing basket and onto a sheet of parchment paper or a silicone bread sling. Score and load into your baking vessel and oven.

Bake for 12 minutes, with the option to uncover and re-score the dough at the 7-minute mark and put the lid back on afterward.

At the 12-minute mark, reduce the oven temperature to 375°F. Bake for 30-45 minutes more or until the internal temperature of the bread is over 205°F. Keep the lid on the entire time except as needed to use a probe thermometer toward the end of the bake.

Let the bread cool for several hours before you slice it.