Glossy, fluid and chifa-style, with pineapple, vinegar, ketchup and light soy sauce. The sweetness does not flatten it: acidity keeps it sharp and cornstarch gives just enough body for glazing, dipping or waking up fried food.
Measure the ketchup, pineapple juice, white vinegar, sugar, soy sauce and salt. Place them in a saucepan and mix until the sugar starts dissolving.
Bring the mixture to a gentle boil and cook for 4 to 5 minutes, stirring now and then, until even, glossy and no longer raw-smelling from the vinegar.
Dissolve the cornstarch in cold water and add it little by little to the hot sauce. Cook for 1 to 2 minutes, stirring, until it coats the spoon in a thin layer.
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Turn off the heat and let it rest for 5 minutes. The sauce should be glossy, fluid and lightly thick; it will thicken a little more as it cools.
