Are you hungry? Only I know a great pizzeria.
Or should I make it one complete sentence and use only as a (coordinating conjunction?)?
Are you hungry, only I know a great pizzeria?
Only can be used for a concession or contradiction.
I would have gone to the party, only I didn't know where it was.
But in the pizza sentence, only makes an offer, so I'm unsure whether it is deemed correct, and what part of speech it is.