"Io non mangio frutta, bensì verdura."
Just for the information of those of you learning English- we would never say this. It would be "I don't eat fruit, but I do eat vegetables".
What Elena18 is pointing out is a more complete sentence which is surely easier to understand, but there are certain speakers of English who would consider the phrasing in the original sentence perfectly acceptable, so don't fear it :)
I find all the translatio of bensi totally awkward. English will never say it this way "but vegetable" ....it's bad english!
I really think of bensì as "but rather". So, "not the apple, but rather the milk" (non la mela bensi il latte). In general, bensi is used only when saying "not this, but rather that". So if you see a sentence in english that has the word "but", and could sensibly be changed to "but rather", use bensi. For the rest (and most) of uses of "but" ("I don't want to, but I will" etc.) use "ma"