I have my suspicion on who that short monk is.
Anyway, is there really mushroom that tenderize meat?
All I know is pineapple .
Don't think there are any commonly used mushrooms that do- dunno if it even could since loosely available enzymes would act on its own body, destroying itself? Certainly does impart flavor though. Who knows with isekai mushrooms though =D
Beyond pineapple, papaya (papain enzyme), and actually a number of other fruits and even onions can release enzymes that do work on meat proteins. Depends on the effect desired, since overexposure and amount makes meat really mushy. Again, isekai mushrooms being able to transform a tough piece of meat (beef I'm guessing? maybe a piece of round?) into delicious steak is quite the magic feat. Only thing that may come close in the world is maybe qu/koji rice mold paste applications- plenty of experiments with doing that to tough cuts of meat, quick/faux "dry-aging", or even long dry curing/aging to mimic steakhouse, or even shorter term with burgers and game poultry for flavor boosting umami and component breakdown.