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1
Build the leaven: combine the starter, flour, and water in a clean jar. Stir thoroughly, cover loosely, and leave at room temperature for 8–12 hours. The leaven is ready when it has doubled in volume, smells yeasty and mildly sour, and a small spoonful dropped into water floats — this float test is the only reliable signal that the culture holds enough gas to leaven the dough.
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2
Mix the dough: combine the bread flour and wholemeal flour in a large bowl. Add 275 ml of the water and mix until no dry flour remains. Cover and rest 30 minutes (autolyse). Add the leaven and salt, then the remaining 25 ml of water. Squeeze and fold the mass until the additions are fully incorporated and the dough feels cohesive and slightly tacky — not sticky and not dry.
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3
Bulk fermentation and folding: leave the dough at room temperature (ideally 24–26 °C) for 3–4 hours. During the first 2 hours, perform a set of four folds (pick up one side, stretch it upward, fold it over the centre; rotate the bowl 90° and repeat for all four sides) every 30 minutes — four sets in total. After the final fold, leave the dough undisturbed. Bulk fermentation is complete when the dough has grown by 20–30%, the surface appears slightly domed and bubbled, and the texture feels airy rather than dense when gently pressed.
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4
Shape: turn the dough onto an unfloured surface. Using a bench scraper, fold the edges toward the centre, then flip the dough so the seam is underneath. Drag it gently across the surface to build tension in the outer skin. The surface should feel taut and hold its round form without spreading — any slack or tearing indicates insufficient tension.
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5
Cold fermentation — this is where the dish lives or dies. Place the shaped dough seam-side up in a well-floured banneton or bowl lined with a floured cloth. Cover and refrigerate for 12–48 hours. The cold arrests the yeast but allows Lactobacillus bacteria to continue generating lactic and acetic acids; the length of this retard directly determines the depth and character of the tang. A 12-hour retard yields a mild sourness; 48 hours produces the pronounced, layered acidity definitive of San Francisco sourdough. Do not shorten this stage.
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6
Preheat and bake with steam: place a lidded Dutch oven in the oven and preheat to 250 °C for at least 45 minutes. Remove the dough from the refrigerator, invert onto a sheet of baking paper, and score the surface with a single deep slash or an ear cut using a lame or sharp blade. Lower into the Dutch oven, cover with the lid, and bake at 230 °C for 20 minutes. The lid traps steam from the dough itself, delaying crust formation and allowing full oven spring — the loaf should visibly surge upward within the first 10 minutes.
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7
Finish and test: remove the lid and continue baking for a further 20–25 minutes until the crust is deep mahogany and the loaf sounds hollow when tapped firmly on the base. Transfer to a wire rack and leave to cool for a minimum of 1 hour before cutting. The crumb continues to set as it cools; cutting early compresses the open structure. When torn, the crust should crackle audibly.