Prepare your levain by mixing active sourdough starter with bread flour and water. Let it sit at around 78°F for 3 to 4 hours until bubbly and doubled in size.
In a large bowl, combine 100 grams of levain with 350 grams of water, bread flour, and whole wheat flour. Mix until combined and let rest for 30 minutes.
Add salt and the remaining 25 grams of water, mixing by hand until well incorporated.
Perform four sets of stretch and folds over the next two hours, waiting about 30 minutes between each set.
After the second stretch and fold, add the drained jalapeños and cheddar cheese, incorporating them gently.
Let the dough rest covered for about 1.5 to 2 hours until puffed and bubbly.
Turn the dough onto a floured surface and shape it into a loose ball. Let it rest uncovered for 30 minutes.
Shape the dough into a tight round and place it seam side up in a floured proofing basket. Remove any jalapeño pieces that might poke out.
Cover the dough and refrigerate overnight (12 to 20 hours) for cold fermentation.
Preheat your oven to 500°F with a Dutch oven inside for at least 30 minutes.
Turn the dough onto parchment paper, score the top with a sharp knife, and place it into the hot Dutch oven. Cover, reduce to 450°F, and bake for 25 minutes. Remove the lid and bake for another 20 minutes until deep brown.
Let the bread cool fully before slicing.
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Notes
Store in a clean kitchen towel or paper bag at room temperature for up to 3 days. For longer storage, slice and freeze in an airtight container.