The g'mara on today's daf discusses the prohibition of cooking on Shabbat.
If a man placed meat on the coals on Shabbat and it was roasted, he is
liable. How much has to be cooked? After discussion about whether it
has to be cooked through or only on one side to count, Raba said that if
it was roasted in one spot the size of a dried fig that is sufficient.
There is further discussion in which either Raba or Rabina said that
if it is roasted in two or three spots that together add up to the size
of a dried fig, that too is a violation -- it doesn't have to be a single
area. Rabina then compares this to boring a hole on Shabbat, which is
prohibited regardless of the size of the hole, but the difference in
these cases is not addressed here. (57a)
I had assumed that any amount of cooking would be prohibited.
Of course, this might not be the last word on the subject.