My Mac Mini w/ Fusion Drive (that’s a hard disk and an SSD pretending to be a single volume for better performance) froze and wouldn’t boot. Nothing would make it boot normally again. Recovery mode was OK but couldn’t erase the boot volume nor mount it via Disk Utility (I just got “Unable to delete the core storage logical volume”), so reinstalling seemed impossible.
Fortunately, I figured out how to do it, using some help from a StackExchange post I found. See below for the details.