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1
Place the kombu in 1.2 litres of cold water in a medium saucepan. Bring slowly to just below a simmer over medium-low heat. Remove the kombu at the first sign of bubbling — the water will carry a faint marine sweetness and a pale golden tint.
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2
Bring the kombu liquor to a full 100 °C. Add the katsuobushi all at once, remove from the heat immediately, and allow to steep for 3 minutes. The dashi is ready when the flakes have settled and the liquid is clear, amber, and fragrant with a clean oceanic note.
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3
Strain the dashi through a fine-mesh strainer without pressing the katsuobushi. Reserve 1.2 litres of clear stock. The dashi may be refrigerated at this stage for up to three days.
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4
Return 1 litre of dashi to the saucepan. Add the tofu and bring to a gentle simmer over medium heat until the tofu is heated through, approximately 2 minutes. Reduce the heat to low so that the liquid holds just below a boil.
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5
Combine the white miso and red miso. Place a ladleful of warm dashi into a small bowl or directly into the ladle itself. Add the blended miso and work it against a fine-mesh strainer or the side of the bowl until fully dissolved — no lumps remain. Add the dissolved miso to the saucepan in a slow, even stream, stirring gently. This is where the dish lives or dies: the pot must not return to a boil from this point forward. Boiling destroys the enzymes, eliminates the volatile aromatic compounds, and produces a flat, harsh flavour that no subsequent adjustment can repair.
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6
Add the sliced negi. Hold the soup at the lowest possible heat for 30 seconds only. Taste for seasoning — the soup should carry a rounded, immediate umami with a faint sweetness from the white miso underlying the depth of the red. Adjust with a small additional quantity of dissolved miso if needed, bearing in mind the topping's own saltiness.
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7
Ladle immediately into warmed bowls and serve at once. Miso soup deteriorates within 30 minutes of preparation — all volatile aromatic compounds are lost on standing.