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1
Place the Rishiri kombu in a clean cold-infusion vessel with 1 litre of cold water. Cover and refrigerate overnight — a minimum of eight hours. The kombu will hydrate and release glutamates slowly, without heat, yielding maximum clarity. At the end of infusion, the liquid should be pale, faintly golden, and entirely transparent.
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2
Transfer the vessel to the stove. Warm over the lowest possible heat, monitoring the temperature with a thermometer. Remove the kombu before the liquid reaches 65 °C — well before any possibility of sliminess. This is where the dish lives or dies: a single degree too many, a moment too late, and the released mucilaginous compounds will cloud the soup irrecoverably. No amount of subsequent straining will restore true clarity.
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3
Strain the dashi immediately through a muslin-lined strainer into a clean pan, pouring gently and without pressing the cloth. The liquid must fall freely; any pressure forces particulate matter through the muslin and destroys the clarity that defines suimono.
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4
Season the strained dashi with fine sea salt and usukuchi soy sauce in the smallest quantities that produce a barely perceptible saltiness. Taste: the seasoned dashi should prompt a quiet sense of satisfaction — round and complete — without registering as salty. It must allow whatever decorative ingredients are placed in the soup bowl to speak without competition.
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5
Ladle into warmed bowls immediately. The dashi must be used within hours of preparation; it does not hold. The finished soup should be perfectly transparent and pale amber, carrying a clean oceanic minerality and a subtle sweetness that is visible in the liquid itself.