Just playing with words while cooking.
sage = an herb, also wise/wisdom
measures = an amount for cooking, also action taken/words spoken
a stewing plot needing to be spoiled by sage measures could =
a story about a plot to kidnap a wealthy restaurantier, a plot to steal a famous recipe, or a plot to poison an important person or a plot to ... in which the chef (who, of course, would also have studied law enforcement in another life) would save the day.
Of course, there are many kinds of stewing plots, lots of things brewing that we could write about.