Mix the Dough: In a sturdy mixing bowl, combine lukewarm water and active sourdough starter. Break up the starter into small, shaggy pieces using your hands or a spatula.
Add Flours: Add whole wheat flour and white flour. Mix until all ingredients are incorporated, and there are no dry bits. Cover the bowl and let the dough rest for 1 hour (autolyse).
Add Salt: Sprinkle salt on top of your dough with a splash of water. Incorporate by hand by squeezing and mixing until all salt is dissolved and incorporated. Cover the bowl and let the dough rest for 1 hour.
Stretch-and-Fold: After the rest, perform the first round of stretch-and-folds, gently stretching the dough and folding it over itself until the dough becomes tight (4-8 times).
Second Stretch-and-Fold: Repeat the stretch-and-fold process for a total of 2 rounds.
Bulk Fermentation: Allow the dough to rest until it doubles in size. This can take anywhere from 4-12 hours, depending on the ambient temperature (longer in a cooler environment, shorter in a warmer one).
Shaping & Bench Rest: After the bulk fermentation, turn the dough out onto the counter and shape into a round ball and let it rest on the counter for 30 minutes. If the dough doesn’t feel gassy or relaxed after this time, allow it to rest for an additional 30 minutes.
Final Shape: After resting, tighten up the round dough ball once again by rounding it on your surface, creating tension. Place your tightly shaped dough ball upside down (seam side up) in a lined 10" proofing vessel.
Cold Proof: Cover the loaf and refrigerate for 8-24 hours for a cold proof.
Bake the Bread: Preheat a Dutch oven in your oven for 1 hour at 500°F (260°C).
Score: Once preheated, flour the bottom of your loaf and carefully flip onto parchment paper or a silicone mat, lightly flour the surface, and score the top.
Insert Loaf: Remove the Dutch oven from the oven, place the scored loaf inside, cover with the lid, and return it to the oven.
Bake: Bake at 500°F (260°C) for 20 minutes, then remove the lid and bake for another 10-15 minutes at 425°F (220°C) until the crust reaches your desired colour.
Cool: Remove the loaf from the Dutch oven and place it on a cooling rack. Allow the loaf to cool for at least 1 hour before slicing.
Enjoy: Once cooled, enjoy your mostly whole wheat sourdough bread with a generous amount of butter!