At startup get nothing but an EFI option which goes to a blank screen - and repeats

[iMac'd]

This followed an aborted Linux Mint install which seems to have wrecked the boot management; using Option at Startup allows for installing from a flash drive and Recovery also shows up; the disk with two OS's on it show as Format unknown on the partition pictograph in Disk utility. Have tried all First aid etc. Any other options or should I proceed with the re-install via flash drive (it's an OCLP installer and this is how I got access to the Disk utility)?

 

[Bigwaff]

Not exactly clear what you are attempting... are you trying to boot into Recovery? If no Recovery, you will need macOS USB install media, either supported macOS install or OCLP macOS install. Boot USB and use Disk Utility to zero the disk. Then reinstall.