Fusion Drive uses both the SSD and HDD to store data. When the Fusion Drive is split, if you create a disk image of the SSD, it can't be used to restore the whole disk drive. If you have Time Machine backup, you can restore after fixing the split Fusion drive, otherwise, recover data from the unbootable Fusion Drive with data recovery software.
The data recovery software is your only choice as you don't have a backup. You can choose iBoysoft Data Recovery for Mac to recover Fusion Drive files from an iMac that won't boot up in macOS Recovery Mode. You will need to run a command in Terminal to install and load the software from its server and click Fusion Drive from the sidebar to continue.
After recovering data, you can proceed to fix the split Fusion Drive to make it bootable again.
In any other cases in which you need to recover data from Fusion Drive, try iBoysoft Data Recovery for Mac.