Sprig of wolfsbane
Wolfsbane is a vegan food, and is also suitable food for herbivorous pets.
Priests begin the game with 1 or 2 sprigs of wolfsbane.
Eating a cursed sprig of wolfsbane might interrupt your meal, so that half of the sprig remains uneaten. If you restart the meal it will nevertheless grant you relief from lycanthropy.
This is in fact a major case of NetHack is not real life, because wolfsbane in real life is very poisonous (so don't eat it in real life), and the name comes from the fact that it was used to poison wolves. Wolves were often killed by Christians for two reasons: first, wolves sometimes prey on livestock and were considered a pest; second, wolves were considered sacred by Germanics (for example Wotan is accompanied by the wolves Geri and Freki), and the monotheist Christians desecrated their values by killing them (or chopping down old oaks which were used to worship Tyr).