Oyster sauce is a thick, glossy, dark-brown Cantonese condiment built on the concentrated savoury liquor of cooked oysters, simmered down with sugar, salt and a soy-and-starch base into a sweet-savoury umami glaze. It is the workhorse seasoning of southern Chinese stir-fries, lending sheen, body and depth far more than any obvious taste of the sea.