And some more
info fun:
www.youtube.com/watch?v=GM1CrR4qVyUDICED ONIONS
Diced onions do not contain as much water as grapes; however,
the steam pressure in them will be ejected from between the onion
layers forming a connecting conducting plasma similar to that in the
grapes. In the case of diced onions, depending on their size,
several chunks would need to be electrically aligned in a random manner
to form an antenna for arcing and burning.
These antennas will not generate as great an internal electric
field within the onion chunks as in the sparking grapes, but the effect
is similar. The onion chunks form a dielectric transmission line and
the interconnected hunks form a sort of resonant antenna to build up
the large fields required for arcing and burning>
Taken from here:
http://www.madsci.org/posts/archives/2000-09/968259641.Ph.r.html