Filming Portrait whilst holding phone in landscape

Does anybody know how I can film in Portrait mode whilst holding the iPhone in landscape mode. I have a stabiliser I use to film videos that holds the iPhone in a horizontal/ landscape position, however the footage is for an app that requires the footage to be in portrait mode. The stabiliser can't hold the phone in portrait mode.

Can I do this? Film portrait whilst holding the camera in landscape position? Is there a setting or app?

Thanks!