How can I reclaim free space on my iMac's Fusion Drive?

ThatPhoenix wrote:

I accidentally created a partition while trying to install windows (not via bootcamp), so I attempted deleting that partition via Disk Utility, but I was greeted with an internal state error. So I deleted it through terminal, but now I'm greeted with only 300 GB available to MacOS. How can I reclaim that missing 700 GB and add it back to Macintosh HD?

https://discussions.apple.com/content/attachment/2df80f1a-a036-4d88-bf12-c0d5e4a0bfcf

MacOS Version: Ventura 13.2.1

From Your Disk Utility you get a much better look VIew>Show All Devices

Are you saying now you have two Containers...?

if in doubt post the out put from the Terminal.app, copy and paste:

diskutil list internal

Volumes can be added or removed in a single Container without penalty.

Add, delete, or erase APFS volumes in Disk Utility on Mac

if that is not your case then reference—this is your way forward:

How to fix a split Fusion Drive - Apple Support

You may well have to erase,reformat, initilize the drives as new and then rebuild the Fusion drive, reinstall the macOS, restore your data from backup. It seems Terminal command-line does this for you in the Help Topic above.