Who is Paul talking to when he says 'you who desire to be under the Law'?
Paul is addressing former pagans who had been transformed by his Gospel but were now listening to false teachers and wanting to do something in addition to the Gospel. They desired to be under the Law, so Paul asks if they do not hear the Law and takes them back to the Old Testament.
Les explains that Paul is writing to believers, formerly pagans, whom he had led to salvation, but who were now troubled by false teachers. They were falling for the idea that they needed to do something in addition to the Gospel. So Paul addresses those who desire to be under the Law, asks if they hear the Law, and, as Les notes, whenever Paul uses that term he takes his readers back to the Old Testament.
Tell me, ye that desire to be under the law, do ye not hear the law?