Restore the machine from a Time Machine backup (NOT a fresh install).See here for details, and also see some background theory.

For example if the HDD is ' disk1', the large partition is probably something like ' disk1s2'.

EXCEPT, in order to preserve the recovery partition on the new setup, do not choose the entire HDD for the Fusion terminal commands, just the large partition of it for storing data.

Re-create the Fusion drive using these instructions from Apple.

Everything should be like a typical system, except the parts of the Fusion drive will appear as two separate disks.

Boot the computer as normal and do the basic setup steps as per any new Mac.Perform a fresh macOS install onto the HDD (not SSD) so that it stays as an jhsf+ (macOS extended journaled) volume.Split up / sever the Fusion drive by following this guide from Macworld.Boot up from the drive by starting the computer and holding down the option key.I used this link to download High Sierra from the App Store, then these instructions from Apple to make a bootable drive. Create a High Sierra installation disk.You can now quit Terminal and eject the volume. When Terminal says that it's done, the volume will have the same name as the installer you downloaded, such as Install macOS Monterey.After the volume is erased, you may see an alert that Terminal would like to access files on a removable volume.Terminal shows the progress as the volume is erased.

When prompted, type Y to confirm that you want to erase the volume, then press Return.Terminal doesn't show any characters as you type. When prompted, type your administrator password.If the volume has a different name, replace MyVolume in the command with the name of your volume. Each command assumes that the installer is in your Applications folder, and MyVolume is the name of the USB flash drive or other volume you're using. Type or paste one of the commands below into Terminal, then press Return to enter the command.Open Terminal, which is in the Utilities folder of your Applications folder.Plug in the USB flash drive or other volume that you're using for the bootable installer.

Mac os high sierra clean install