Comment 1 for bug 1590668

> I unsuspectingly choose that option, as I wanted to install the
> 3rd party support options.

If you were presented with this option, it was because you had already selected to install third-party drivers that would require disabling Secure Boot.

> Further research showed me that all the OS needs is for Secure Booting
> to be turned off in BIOS, so, erm... why didn't they just get the user to do
> that ahead of time

Because there is no consistent user interface for disabling Secure Boot in the firmware, making this impossible to provide clear directions for all users to follow.

> Anyway, my question: Now that I have this "Booting in insecure mode" message
> in my boot process, how do I get rid of it. I have of course tried reinstalling a
> number of times with Secure Boot in BIOS on and off.

If you want to re-enable SecureBoot validation within shim, you can do so by running this command, then rebooting to confirm the change:

  sudo mokutil --enable-validation

However, doing so will render any third-party kernel drivers (e.g., nvidia video drivers) inoperable on your system which may result in a degraded experience.