Secure erasing data on a Mac with HDD running Catalina

It took me a lot of searching to find the answer. I'm reposting what I found so that it might be easier for the next victim.

Reboot the iMac and hold down

Command R

Once you are in the Recovery Partition, open Disk Utility.

From the view menu at the top select "Show All devices".

On the disk utility menu select the top level drive above Macintosh HD (APPLE SSD XXX). Click erase.

Or you can just select Macintosh HD and click erase, make SURE you click "Erase Volume Groups"

When complete all data will be gone, you can quit out of Disk utility and then can select Install macOS.

(Courtesy of ClassicII)