Oyster sauce

Oyster extract thickened and sweetened. Its value is the 5-prime nucleotides it carries, IMP and AMP, rather than its salt. The umami receptor T1R1/T1R3 binds glutamate at one site and is allosterically stabilised by nucleotides at another, so oyster sauce alongside a glutamate source produces a response well above the sum of the two. That is the reason to keep it in an already salty dish: it multiplies what is there rather than adding to it. Check the label for actual oyster extract; the cheapest jars are caramel and MSG with a picture of an oyster.