In the evening, combine flours and salt in a large bowl. Add 150g active sourdough starter and the water.
Mix with a fork or your hands until a shaggy dough forms with no dry flour remaining.
Cover with a damp towel or plastic wrap. Let ferment at room temperature for 8–12 hours (overnight).
The dough is ready when it has roughly doubled in size and the surface is bubbly.
Turn the dough onto a floured surface. Fold it over itself a few times to shape into a round. Place seam-side down in a floured proofing basket or lined bowl.
Let proof for 1–2 hours while you preheat the oven to 450°F (230°C) with a Dutch oven inside.
Turn the dough into the hot Dutch oven. Score the top with a sharp knife or razor blade.
Bake covered for 30 minutes, then uncover and bake 15 minutes more until deep golden brown.
Cool completely on a wire rack before slicing — at least 1 hour.
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