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1
Place the konbu in cold water in a small saucepan. Bring slowly to just below a simmer over medium-low heat; remove the konbu when the water reaches approximately 60°C and small bubbles begin to rise from the base of the pan. Raise the heat, bring the liquid to a full simmer, then remove from the heat. Add the katsuobushi in a single addition; steep undisturbed for 2 minutes. Strain through a fine-mesh cloth without pressing. The resulting ichiban dashi must be pale gold and fully transparent — any cloudiness indicates the konbu was boiled or the katsuobushi was agitated.
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2
Pour the mirin into a small saucepan and bring to a gentle simmer over medium heat. Hold at a simmer for 1–2 minutes to cook off the alcohol. Do not reduce significantly — the mirin should remain fluid and pale, not syrupy. This is where the dish lives or dies: over-reduction at this stage creates excessive sweetness that will overwhelm the clarity of the dashi in the finished tentsuyu, and cannot be corrected.
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3
Combine the simmered mirin, soy sauce, and ichiban dashi in the saucepan. Warm together over low heat, stirring once to blend. Bring to approximately 60°C — the sauce should steam gently and feel warm but not hot when the bowl exterior is held briefly. Do not boil: boiling will drive off volatile dashi aromatics and deepen the soy beyond the intended register.
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4
Ladle the warm tentsuyu into individual small, wide porcelain bowls chosen for heat retention. Serve immediately alongside freshly fried tempura. Present grated daikon and grated ginger separately for each diner to add to their own bowl — pre-mixing dilutes and cools the sauce and is a common error. The sauce at the table should still steam faintly: this signals correct serving temperature.