Mix and Rest Dough:

In a stand mixer bowl, mix the bread flour, active sourdough starter, sugar, and lukewarm water on low speed until just combined; the dough will be shaggy and a bit dry. Cover the bowl with a damp tea towel and let it rest at room temperature for 30 minutes. Tip: Set the measured salt on the towel as a reminder to add it next.

Knead with Salt:

Add the salt and knead on low for 5 to 9 minutes, until the dough is smooth and forms a ball that cleans the sides and bottom of the bowl. If the dough seems too dry, add a bit more water; if too wet, add a bit more flour.

First Rise:

Form the dough into a smooth ball and place it in a clean, lightly oiled bowl. Cover with greased plastic wrap and set in a warm area to rise for 3 to 4 hours, until doubled in size and very puffy.

Divide and Shape:

Divide the dough into two equal portions. Shape each portion into a smooth ball by gently pulling the dough from underneath and tucking it inward until the surface is taut.

Cold Proof Overnight:

Place each dough ball into a flour-dusted proofing basket or onto a parchment-lined baking sheet. Cover with greased plastic wrap and refrigerate overnight, or for up to 18 hours.

Bake in Dutch Oven:

Preheat the oven to 425°F (220°C) with an empty Dutch oven inside. Score the top of each loaf with a lame or serrated knife. Transfer one loaf onto a piece of parchment paper and carefully lower it into the hot Dutch oven. Cover with the lid and bake for 25 minutes, then remove the lid and bake for another 20 minutes, until deep golden brown.

Cool Before Slicing:

Take the loaf out of the Dutch oven and set it on a cooling rack. Let it cool for at least 4 hours before slicing. Store at room temperature in a bread bag or airtight container for up to 4 days.