UEFI not recognising EFI partition: no booting options

Forget grub entirely - it is nothing but a distraction. It isn't even a boot-loader anymore; on EFI systems the bootloader is built-in to the firmware. grub is just a boot-manager in that context - and almost definitely entirely redundant. What's more - it is probably the grub install that broke everything in the first place.

These are the things you need:

1. A FAT-formatted GPT partition of type ef00.
2. A UEFI-compatible system kernel located on that partition (such as the linux kernel).
3. The path to that system kernel saved to a UEFI environment variable (commonly Boot0000-{UUID}, but this also depends on the value of BootOrder-{UUID}).

Strictly speaking, that is all. You can achieve the above setup with nothing more than gdisk and the efibootmgr command-line tools.

Pragmatically, a boot-manager does make sense - but grub is the most complicated of all of those available. As is elsewhere recommended, rEFInd is probably the best of the bunch.

I have written a step-by-step tutorial before on how to partition, format, and setup a rEFInd-enabled EFI system partition before here. Here also is another answer on this subject, in which you might find some further explanation about the assertions I make here.

There's another way: you can create a menu entry that tells GRUB to load another secondary grub.cfg, such as one from another Linux distro.

For example, I started with Gentoo Linux from which I installed GRUB2 into the MBR (the machine is too old for EFI).

I then installed NixOS, which I configured to generate grub.cfg in it's own /boot (separate from Gentoo's /boot) but without installing GRUB.

To clarify, grub-install was executed from Gentoo but not from NixOS.

Next, to be able to boot NixOS, I added this to /etc/grub.d/40_custom in Gentoo:

#!/bin/sh
exec tail -n +3 $0
# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.

menuentry 'NixOS' --class gnu-linux --class gnu --class os $menuentry_id_option 'nixos-0aca58bc-8fdb-4a07-aa2f-56406bcf19b7' {
        set root='hd0,msdos4'
        configfile /nixos/root/boot/grub/grub.cfg
}

The key is the configfile /nixos/root/boot/grub/grub.cfg line. It tells GRUB to load another grub.cfg. I then ran grub-mkconfig from Gentoo to apply the changes.

Now, when I boot and select NixOS the entire GRUB interface refreshes to reflect the NixOS grub.cfg, from which I can boot the OS. Unlike chainloading, this configuration utilizes a single installation of GRUB; it simply uses a second configuration.