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covered, 20–22°C, overnight

sourdough starter + 200 g of the rye flour + 200 ml of the water, stirred to a thick batter and left until domed, bubbled, and sharply sour = sponge. The souring is not flavour decoration: rye's alpha-amylase survives into the bake and keeps cleaving starch while the loaf sets, and only a dough acidified to roughly pH 4.3–4.5 shuts it down in time. An under-soured rye bakes gummy no matter how long it stays in the oven.