Remove top cap and rough bottom end of each king oyster mushroom and cut remaining stalk into approximately 3/4" or 2 cm thick slices. Keep the top caps for another favourite mushroom dish!
Cut a diamond cross-hatch pattern in the top and bottom of each 'scallop', being careful not to cut too deeply and set aside while preparing marinade.
Combine all marinade ingredients in a small sauce pot and bring to a boil before reducing to a simmer and covering for about 10 minutes. Let cool completely before pouring into a bowl just big enough to hold all of the 'scallops' and the marinade.
Add the 'scallops' to the marinade bowl and place a smaller bowl or plate (that fits inside the bowl) on top of them to make sure they are submerged in the marinade. Leave on the counter for 3 to 4 hours.