In a large bowl, mix the bread flour, whole wheat flour, and 350g of the water until just combined. Cover and rest for 45 minutes at room temperature; the dough should look slightly hydrated and shaggy.

Add the active sourdough starter and the remaining 25g water, then squeeze the dough between your fingers until fully incorporated. Mix again after adding fine sea salt, then let rest for 30 minutes; it should feel smoother and slightly elastic.

Coat your hands with olive oil, then perform the first set of stretch and folds. Repeat for 2 hours total with 30-minute intervals between sets.

On the second stretch-and-fold set, laminate the dough flat and scatter the sharp cheddar cheese and medium jalapeños evenly before folding them in. Continue folding so the mix-ins distribute through the dough and it becomes more cohesive.

Complete the remaining stretch-and-fold sets to finish the 2-hour window. The dough should look stronger, puffier, and hold shape between folds.

Cover the dough and bulk ferment at 75–78°F for 6–10 hours, until it has risen 50–75% and is bubbly and domed. Aim for a noticeably expanded dough volume with visible fermentation bubbles.

Turn the dough onto an unfloured surface and pre-shape into a round, then bench rest uncovered for 20–30 minutes. The surface should relax and look slightly drier as it rests.

Final shape into a tight boule or batard, then place seam-side up in a floured banneton or a bowl lined with a floured kitchen towel. Cover so the dough doesn’t dry out.

Cold proof in the refrigerator for 12–16 hours (or overnight). The dough should feel airy and look expanded right before baking.

One hour before baking, place the Dutch oven in the oven and preheat to 500°F (260°C). The pot should be fully hot for maximum oven spring.

Turn the dough out onto parchment paper and score the top with a sharp lame or razor blade, using a long curved slash. A clean, decisive cut will open the loaf as it bakes.

Carefully lower the dough (with parchment) into the hot Dutch oven and bake covered at 500°F for 20 minutes. Look for strong rise and early crackling around the score line.

Remove the lid and reduce heat to 450°F (230°C), then bake uncovered for 20–25 minutes. The loaf should become deep golden brown with a dry, crackly crust.

Cool the loaf on a wire rack for at least 1–2 hours before slicing. The crumb should set and feel less gummy when pressed.