Use Pureboot recovery shell to set up boot from USB?

I was going about reinstalling PureOS on my Librem Mini and I think I - whoops! - recreated my MBR on an encrypted partition. The result is that Pureboot can’t mount the /boot directory and keeps throwing me back into the recovery shell.

I’d like to restart from my USB drive, and effect a do-over, properly partitioning my drive this time. But to do that I need to get to my USB device, which has my PureOS installer S/W on it.

Can anyone point me to some documentation about Pureboot that could guide me on how to use my recovery shell to point Pureboot back at the USB drives?