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1
Combine the wheat flour with roughly 400 ml of the measured water in the mixing bowl. Mix briefly to a shaggy mass. Cover and leave to autolyse for 30 minutes. The purpose is to give the wheat gluten a head start on development before the rye flour is introduced.
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2
Add the rye flour, the twice-refreshed rye levain, the salt, and any remaining water to the autolysed wheat dough. Mix at moderate speed or by hand until the mass is cohesive and the ingredients are fully incorporated. Do not over-mix: the dough is done when it holds together and leaves the bowl sides, but remains slack, slightly sticky, and without the taut windowpane associated with pure wheat doughs. Over-mixing at this point damages the rye pentosan gel structure and cannot be corrected.
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3
Transfer the dough to a lightly oiled container. Conduct bulk fermentation for 2–3 hours at room temperature, performing a series of stretch-and-fold cycles during the first hour to build structure. Alternatively, retard the dough overnight in a refrigerator. Bulk fermentation is complete when the dough has increased visibly in volume and holds a slight dome.
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4
Turn the dough onto an unfloured surface. Shape with wet hands — not floured hands — into a large round or oval of approximately 1.5 kg. Wet hands prevent tearing and preserve the surface tension of the slack dough. Transfer seam-side up into a well-floured banneton.
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5
Proof in the banneton for 60–90 minutes at room temperature, covered. The loaf is ready to score and bake when it has risen modestly and the surface, pressed lightly with a wet finger, springs back slowly and only partially.
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6
Heat the oven to 240°C with a baking stone or heavy tray in place and a vessel for steam. Turn the loaf onto the stone, score with moderate cuts — the 50% rye content limits the dough's capacity to support deep slashes — and introduce steam immediately. Bake for 15 minutes at 240°C, then reduce to 210°C and continue for a further 35–45 minutes. The loaf is baked when the crust is deep mahogany-brown, the base sounds hollow when knocked, and an internal temperature reaches approximately 98°C.
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7
Transfer to a wire rack. Allow the loaf to cool completely before wrapping. Do not cut before day 2: the crumb requires this period to set fully and the flavour reaches its peak on days 2–3. The bread keeps for 7–10 days.