Preheat the oven to 425°F (220°C). Place the whole jalapeños on a baking sheet and roast for 15–20 minutes, turning once, until lightly blistered and softened. Let them cool completely, then remove the stems. Peel the skins if desired, remove the seeds and membranes for a milder loaf (optional), and dice into small pieces. Pat dry with paper towels.
In a large mixing bowl, combine the bread flour, water, and active sourdough starter. Mix until no dry flour remains. Cover and let the dough rest for 30 minutes.
Add the salt
Sprinkle the salt over the dough and gently work it in until fully incorporated. Cover and let the dough rest for another 30 minutes.
Over the next 3½ to 4 hours, perform 4 sets of stretch and folds, spaced about 30 minutes apart.
After the first set of stretch and folds, gently turn the dough onto a lightly dampened work surface. Carefully stretch it into a large rectangle, being careful not to tear the dough.Evenly sprinkle the roasted jalapeños and cheddar cheese over the surface. Fold the dough over itself, enclosing the fillings, then return it to the bowl. Perform 3 more sets of stretch and folds, spaced about 30 minutes apart, to strengthen the dough and help distribute the cheddar and jalapeños evenly. After the final fold, allow the dough to continue fermenting until it has increased in volume by about 75%, looks airy, and has visible bubbles around the edges.
Turn the dough onto a lightly floured work surface and divide it into 2 equal pieces. Gently pre-shape each into a round. Cover and let them rest for 20 minutes.
Shape each loaf into a tight boule or bâtard and place seam-side up into lightly floured bannetons.
Cover the bannetons and refrigerate for 12–16 hours.
The next day, place a Dutch oven in the oven and preheat to 475°F (245°C) for at least 45 minutes.
Turn one loaf onto parchment paper, score the top with a bread lame, and carefully transfer it to the hot Dutch oven.
Bake covered for 25 minutes, then remove the lid and bake for another 20 minutes, or until the crust is deep golden brown and the internal temperature reaches about 208–210°F (98–99°C).
Repeat with the second loaf.
Cool
Transfer the loaves to a wire rack and let them cool completely before slicing. This allows the crumb to finish setting and gives you the best texture.