The next morning, check that your starter has noticeably risen and is bubbly and active. For best results, use it at or near its peak.
Add the active sourdough starter and room temperature (or slightly warmed) water to a large mixing bowl and stir to combine.
Add in the bread flour, whole wheat flour, and sea salt flakes. Mix until fully incorporated and a shaggy but rounded dough ball forms. Cover and rest for about 45 minutes.
Perform the first stretch and fold by gently pulling one side of the dough upward and folding it over itself. Rotate the bowl and repeat around all four sides. Cover and rest for about 45 minutes.
Perform a second round of stretch and folds. Cover and rest for another 45 minutes.
During the third stretch and fold, add the sliced, de-seeded jalapeños and thick-shredded cheddar cheese, gently working the inclusions into the dough. Cover and rest for about 45 minutes.
Perform a fourth and final round of stretch and folds, using this round to help distribute the jalapeños and cheddar evenly throughout the dough.
Cover the bowl and allow the dough to bulk ferment at room temperature for the remainder of the day. The dough should noticeably increase in size and look lighter, puffier, and more aerated.
That evening, gently turn the dough onto a lightly floured work surface and allow it to bench rest for about 30 minutes.
Shape the dough into a tight ball, gently creating surface tension across the top of the loaf.
Generously dust a banneton basket with rice flour and place the shaped dough into the basket seam-side up.
Cover the banneton and refrigerate overnight to cold proof for approximately 12–16 hours.
Day 3: Bake
The next morning, place the Dutch oven and lid inside the oven and preheat to 500°F for approximately 30 minutes.
Remove the dough from the refrigerator and gently turn it out onto a silicone bread sling or sheet of parchment paper. Score the top of the loaf with a bread lame or very sharp knife.
Carefully remove the preheated Dutch oven from the oven. Using the silicone bread sling or parchment paper, gently lower the dough into the Dutch oven and cover with the lid.
Return the covered Dutch oven to the oven and immediately reduce the temperature to 450°F. Bake covered for 25 minutes, allowing the oven temperature to gradually come down from 500°F.
Carefully remove the lid with oven mitts and continue baking uncovered at 450°F for another 18 minutes, or until the crust reaches your desired deep golden-brown color.
Carefully remove the bread from the Dutch oven and transfer it to a cooling rack. Allow the loaf to cool completely before slicing so the crumb has time to fully set.