Combine the sourdough starter, bread flour, whole wheat flour and water. Mix until no dry flour remains. Cover and let the levain rise at room temperature until it is bubbly, active and near its peak, about 3-5 hours depending on temperature and starter activity.

Add the bread flour, whole wheat flour and 350 grams water to a large mixing bowl. Mix until all of the flour is hydrated and no dry spots remain.

Cover and let the dough rest for 30 minutes.

This is an autolyse, which allows the flour to fully hydrate and begins gluten development before the levain and salt are added.

Add the ripe levain to the autolysed dough. Work it into the dough by gently pinching, folding and squeezing until the levain is evenly incorporated.

Cover and let the dough rest for about 20 minutes.

You may not need all of the additional water. The goal is a soft, cohesive dough that is tacky but still has enough strength to hold together. If the dough becomes excessively loose or difficult to handle, stop adding water.

When the additional water is added, the dough may temporarily look loose, slippery or even as though it has started to fall apart. This is normal. Continue mixing, folding and working the dough, and it will begin to come back together as the gluten develops.

Cover and let the dough rest for 30 minutes.

After the 30-minute autolyse, add the ripe levain to the dough and mix using the dough hook on low speed for about 3-4 minutes, or until the levain is fully incorporated and the dough looks cohesive.

Cover and let the dough rest for 30 minutes.

Continue with the stretch-and-folds during bulk fermentation as directed below.

Once the dough has rested, begin the bulk fermentation stage.

Perform 3 sets of stretch-and-folds, spaced about 30 minutes apart.

For each set, gently stretch one side of the dough upward and fold it over itself. Rotate the bowl and repeat on all four sides. Avoid aggressively stretching the dough, especially as it becomes more aerated.

After the third set, assess the dough. If it already feels strong and smooth and holds its shape well, no additional folds are necessary. If it still feels loose or lacks structure, perform a fourth set.

Continue bulk fermentation after the final set of folds until the dough has visibly risen and looks noticeably aerated and puffy. You should see bubbles developing along the sides and surface, and the dough should have a soft, pillowy appearance with a gentle jiggle when the bowl is moved.

Because this dough contains 50% whole wheat flour, fermentation can move more quickly than a mostly-white sourdough. Rather than waiting for the dough to double in size, use the appearance and feel of the dough to determine when bulk fermentation is complete.

Turn the dough out onto a lightly floured work surface.

Gently shape it into a loose round, using a bench scraper to help create some surface tension without pressing the air out of the dough.

Let the dough rest uncovered for about 20 minutes.

Lightly flour the top of the dough and carefully flip it over so the floured side is underneath.

Shape the dough into a tight boule, creating enough surface tension for the loaf to hold its shape while still being gentle enough to preserve the fermentation you've developed.

Place the shaped dough seam-side up in a well-floured 9-10-inch (23-25-cm) banneton.

Cover and refrigerate for 10-14 hours.

Place a Dutch oven with the lid on in the oven and preheat to 475°F (245°C) for about 30 minutes.

When ready to bake, carefully turn the cold dough onto a piece of parchment paper.

If there is excess flour on the surface, lightly mist the loaf with water to reduce the floury appearance. Do not saturate the dough.

Score the loaf with a lame or sharp razor.

Carefully transfer the dough to the preheated Dutch oven and cover with the lid.

Bake at 475°F (245°C) for 25 minutes.

Check the internal temperature in the center of the loaf. It should be around 190°F (88°C) or higher before removing the lid. If it is still considerably below 190°F (88°C), continue baking covered for a few additional minutes and check again.

Once the loaf reaches around 190°F (88°C) or higher, remove the lid and reduce the oven temperature to 450°F (230°C).

Continue baking uncovered for about 20-25 minutes, or until the loaf is deeply golden brown and the internal temperature reaches 205-210°F (96-99°C).

For more even browning, you can carefully transfer the loaf to an oven-safe wire rack set over a baking sheet for the final few minutes of baking.

Remove the bread from the oven and allow it to cool completely on a wire rack before slicing. For the best crumb texture, allow at least 2 hours of cooling time.