According to the French dictionnaries, the original meaning (outdated) is "a stew made of several meats". Next is "an incongruous collection". The most common meaning is "a non homogenous collection of musical pieces" - in English a Medley.
Most recent usage being for a single pot with several plants or flowers.
My understanding is that the word Medley has in the same way evolved to other kinds of disparate collections (such as "a medley of flowers").