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1
Combine the yuzu juice, dashi, soy sauce, and warmed mirin in a small bowl or jug. Stir to integrate. Set aside at room temperature. The ponzu should smell bright and citrus-forward with a clean soy undertone — neither sharp nor flat.
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2
Place the kombu in the clay pot (donabe) with the soft water. Allow to cold-steep for a minimum of 20 minutes. Set the pot over a low flame. Remove the kombu just before the water reaches a simmer — the surface will show fine bubbles rising around the kombu and the water will carry a faint oceanic mineral scent. Do not allow the kombu to boil.
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3
Lower the tofu blocks gently into the kombu-infused water. This is where the dish lives or dies: maintain the water at a sustained, very gentle simmer — the surface should tremble and rise in slow, lazy plumes, never at a rolling boil. Vigorous boiling fractures the silken curd and destroys the custard texture that is the entire purpose of the preparation. Heat until the tofu is warmed fully through — 8 to 10 minutes — and the blocks are visibly swollen and quivering at the edges when nudged with a ladle.
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4
Set small individual dipping bowls of ponzu at each place. Arrange garnishes — sliced spring onion, grated ginger, yuzu zest — in separate dishes at the table. Using a wide-mouthed ladle, transfer one tofu block per serving into each bowl, retaining a small amount of the hot water. Spoon a restrained amount of ponzu over the tofu; add garnishes to taste. The tofu should be steaming visibly and yield at the touch of a spoon with no resistance.