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1
Place the beef in a large pot with the water. Bring to the boil over high heat, then skim every trace of grey foam from the surface until the liquid runs clean. Reduce to the gentlest possible simmer and cook, uncovered, until the beef is fully tender. The broth must remain completely clear and golden — not a trace of cloudiness.
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2
While the beef simmers, pound or blend shallots, garlic, and turmeric to a smooth paste. Fry the paste in neutral oil over medium heat, stirring continuously, until fragrant and the raw smell has entirely gone — the paste should deepen slightly in colour and the oil begin to separate at the edges.
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3
Lift the cooked beef from the pot. Slice thinly across the grain. Reserve the broth.
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4
Stir the fried spice paste into the reserved broth. Season with salt. This is where the dish lives or dies: the broth must remain perfectly clear and golden-turmeric in hue. Any vigorous boiling or impure paste at this stage will cloud it irreparably — a cloudy soto Ambengan is, by Surabayan standard, a failed soto. Maintain the lowest possible heat and do not allow the broth to boil again.
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5
Divide the blanched bean sprouts and sliced beef among four warmed bowls. Ladle the hot, clear broth over. Garnish with sliced spring onion and fried shallots. Serve each portion with a halved lime alongside. The cook's test: the broth in the finished bowl should be clear enough to read the bottom of the bowl through it, bright golden from the turmeric, and sharp on the tongue from the lime squeezed at table.