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1
Place kombu in cold water in a medium saucepan. Bring slowly to just below a simmer over low heat — this takes approximately 15 minutes. Remove kombu before the water boils. Raise heat until the water reaches a full simmer, add katsuobushi, and steep for 3 minutes without stirring. Remove from heat and strain through a fine sieve without pressing the solids. The finished dashi should be pale gold, clear, and carry a clean marine umami on the palate.
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2
Return the dashi to a medium saucepan over moderate heat. Bring to a gentle simmer. Add the curry roux pieces gradually, whisking continuously until each piece is fully dissolved before adding the next. The quantity is deliberately conservative: the target is a dashi-forward broth with a curry note, not a curry sauce. Taste after each addition — the dashi character must remain dominant; the curry registers as seasoning, not base. This step decides whether the dish is karē udon or karē raisu in a bowl: if the curry overwhelms the dashi, the dish is already lost.
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3
Assess viscosity. The broth should coat a spoon lightly and have perceptible body without setting or gelling. If the roux alone has not achieved this, introduce the potato starch slurry in a thin stream while stirring, adding only enough to reach a lightly viscous consistency. The broth must flow freely in the bowl; it must not cling in clumps or pour as a gel. Hold the broth at 80°C minimum throughout service.
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4
For butaniku karē udon: add thin-sliced pork belly directly to the simmering broth at service, not before. Cook for 2–3 minutes until the pork is just cooked through and has released its fat and gelatin into the broth. The broth will deepen in colour and richness. The pork should be tender and yielding, not chewy.
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5
Bring a large pot of unsalted water to a rolling boil. Add fresh or chilled udon noodles and cook according to the noodle's own specification — typically 1–3 minutes for chilled, less for fresh. Drain thoroughly. The noodle should be thick, yielding, and fully heated through with no cold centre. Transfer immediately to warmed serving bowls.
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6
Ladle the hot curry broth over the noodles immediately, ensuring the broth reaches at minimum 80°C in the bowl. Place pork belly slices over the noodles. Set an onsen tamago on the broth surface — do not break it; the yolk breaks into the broth during eating and enriches the final third of the experience. Finish with sliced spring onion. Serve without delay: the noodle absorbs liquid progressively and the broth thickens as it cools, producing a dish of different character with each passing minute.