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1
Par-boil the peeled, halved or quartered satoimo in unsalted water for 3–4 minutes, then drain. This reduces excess surface starch that would otherwise make the broth gluey.
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2
Combine the water, soy sauce, and sake in a large donabe clay pot or Dutch oven. Add the kombu. Place the drained satoimo into the cold broth — not boiling — so that the taro absorbs flavour gradually as the liquid heats. This is where the dish lives or dies: satoimo added to already-boiling broth tightens on the outside before the interior can drink in the soy-and-sake flavour, producing a starchy core with a seasoned shell rather than a uniformly flavoured piece.
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3
Bring the pot to a moderate simmer over a wood fire or gas burner. Add the hand-torn konnyaku pieces. The ragged surfaces hold broth better than any cut face would. Simmer until the satoimo begins to yield at the edges — a skewer meets slight resistance at the centre.
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4
Add the thinly sliced beef in loose folds across the surface of the broth. Stir gently once. The beef is done when it has just lost its raw colour and the broth carries a faint iridescence from the rendered tallow — do not prolong cooking beyond this point or the slices tighten.
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5
Add sugar to balance and the green onion lengths. Stir once. Remove the kombu. Taste the broth: it should be savoury and clean, with the earthy sweetness of the taro evident beneath the soy. Adjust with a small measure of soy sauce if required. Serve directly from the pot into bowls.
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6
Imoni-curry-udon transformation — second stage: when the stew is largely finished, stir the curry roux blocks into the remaining hot broth until fully dissolved. The residual beef tallow and taro starch in the broth require no additional seasoning. Add the udon noodles and simmer only until heated through and supple — overcooked udon becomes irreparably mushy. Serve immediately.