How to restore Mac partition I'll preface this question with the obligatory "I did something stupid"...

I wanted to factory restore my 2012 MacBook Pro.  I did Command-r, and later Command-Option-r, but both times Reinstall macOS failed after a while.  (I did not write down the exact failures but, in hindsight,  it may have been connectivity problems).

I then went into Disk Utility and saw Restore.
I guessed that restoring from "OS X Base System" into my main partition would at least take me back to the original OS version and I could upgrade from there.

Apparently, I was wrong.

Regular reboot now takes me into macOS Utilities and Disk Utility shows that my disk now has just one partition, a 500GB "OS X Base System".

I can't do a Reinstall OS since the only partition is in use, and I can't shrink the partition either.

Do I have any way forward from here?  (If it helps: I have other computers here and various USB drives. But I don't have any other Macs).
I was ignorant. I simply needed to use Command-Option-R.