Unfortunately the Apple @KiltedTim linked assumes the drive is an SSD or has Filevault enabled. Apple has neglected to provide proper instructions for securely erasing a hard drive.
Yes, the internal drives can still contain data. I'm guessing the internal hard drive portion of the Fusion Drive is probably bad which is why you are booting from an external? It can make it tricky to make sure the hard drive portion of the Fusion Drive is securely erased since to make sure data on the internal hard drive is gone, you need to write zeroes to the whole hard drive. If the hard drive is bad, then this will likely fail. You can use Disk Utility to perform the secure erase on the hard drive (Disk Utility won't let you do this on a Fusion Drive or the SSD portion).
A simple erase of the SSD using Disk Utility is enough to securely erase the SSD due to how SSDs work.
After erasing the SSD & HD separately, then you can rebuild the Fusion Drive using the instructions in the following Apple article:
How to fix a split Fusion Drive - Apple Support
Then reinstall macOS to the Fusion Drive if you want (assuming the internal drive is functional). If the internal SSD is 128GB, then you can easily install macOS to just the internal SSD and forget about recreating the Fusion Drive if the hard drive portion is bad.