how can I Repair / Rebuild a Fusion Drive on an iMAC?

I do have Applecare if necessary but would like to solve the problem myself if possible.

Did you try any of the reinstall options that Apple describes?
https://support.apple.com/en-us/HT204904

The recovery procedure started at boot with Cmd+R should show you your drive to restore to. Does it?

The solutions that Apple suggest in the link you provided do not work since no drive is visible.
the same applies to the cmd+R.
the issue is probably because the fusion Drive is not functioning correctly

So if it's not visible then, how did you 'delete' it exactly? You did not open the iMac right?

If the drive is not functioning correctly and since your model is late 2015 then you have warranty and should take it to Apple to have the drive replaced.

I probably will take it to Apple - I have Applecare but it would be good to do it myself.
I used the clean install process of using a thumb drive and the disk Utility and in the process of formatting row HD drive i probably inadvertently deleted it

You probably deleted the partition (not the drive), can you post a screenshot of the disk utility? Adding a partition is what you should do first and then start the recovery. You could also use Command-Option-R at startup instead to start recovery from internet.

this something like what happened:
I entered Recovery Mode using command R at boot. Then I went to disk utility (the new one) and clicked on my 2TB HD and clicked on erase. It was something like Fusion Drive > Macintosh HD. I clicked on Macintosh HD, erase button was activated. However it ended up with an error for some reason. I cannot reproduce the error message because now the drive just doesn't appear. Only Fusion Drive on the top, with an option to partition.

I try to partition and it gives me this error :

"Internal Error
Is there another way to rebuild the HD ?

The error on the erase combined with the internal error would mean I'd take the machine to Apple. You still have warranty (even without Apple Care) since the machine is of end 2015. Apple will replace the drive but I'd want to know iiwy how a normal erase command could lead to all this.

I am taking the iMAC to Apple now and will report back their findings to all of you

I collected the iMac from Apple and it is now working perfectly.
They did not replace the Fusion Drive, instead the repaired / rebuilt is using Terminal.
I was given the following link to explain what was done:

https://www.reddit.com/r/applehelp/comments/3n1fiw/deleted_macintosh_hd_in_el_capitan_install_now/

The issue is - how can the MAC HD be deleted so easily on the Fusion drive? since when doing a clean install as previously has been possible, the drive is actually removed in the process.
Apple will have some explaining to do - though my local store ddid not pursue the issue

That's basically the same instruction as one of the steps in the first link I provided.  The fusion drive is kind of a gimmick.  It's two drives in one and Apple decided to merge the two as a single unit to the user.  The link I provided shows you how you can do that with a separate SSD and HD to make it act the same as a single unit fusion drive.

My concern now is how to do clean install of the next new OSX without deleting the drive again

how can Apple allow for the drive being deleted in what has been a standard process for clean installations of new OSX's.
surely it should be more difficult to do this.

Thank you for your comments which were a source of the fault being cured - albeit  by the local Apple store technician on this occasion.
The fact remains that there is an Apple design issue which should prevent such a problem arising - especially when doing a clean install.
How can one do a clean install in the future  if you have an Apple Fusion Drive?

You need to use the Terminal command line diskutil when you want to erase a fusion drive.  The GUI Disk Utility app does not work.

Here's another article about it:
http://macs.about.com/od/diyguidesprojects/ss/Split-Your-Fusion-Drive-Apart.htm

Thank you - I do not think that I will do Clean installs anymore......