Excellent question! I think - this is just a guess, and I hope some native speaker can help us - that German would differ selten-adverb from selten-adjective with declension. In this case, strong inflection (http://en.wikipedia.org/wiki/German_declension#Strong_inflection.5B6.5D.5B7.5D)
So, your sentence would be: Der Koch kocht seltene Gemüse.
Am I right, German people?
Because selten is a time adverb, referring to how frequently the cook cooks vegetables (not very frequently). On the other hand, "few" is (kind of) a number, referring to how many vegetables the cook cooks (not many). Hence it's not a good translation of what the German sentence means.