For a while I had a problem with my boot partition being too small. It was fine for one kernel image but not for two. So whenever kernel was updated it was failing because of lack of space. Five years ago 500MB was enough but not nowadays.
Today I decided to finally fix that.
First, I did created new LVM volume for /boot:
sudo lvcreate -L 1G -n boot root-vg
Then I mounted it and copied contents of old /boot into it
sudo mount /dev/mapper/root--vg-boot /mnt/boot sudo rsync -avp /boot /mnt/boot
After that I edited /etc/fstab and changed old boot partition to new boot partition and then rebooted the machine.
Then I did update Grub:
sudo update-grub
Everything went well so I removed old partition as no longer necessary.
And this time PC failed to start.
Being in Grub emergency shell I managed to boot my machine by doing:
set root=(lvm,root--vg-boot) linux /vmlinuz-6.18.9+deb14-amd64 root=/dev/mapper/root--vg-root initrd initrd.img-6.18.9+deb14-amd64 boot
Which meant that system was fine, but Grub was misconfigured.
I tried to fix that by reinstalling grub (which according to quick web search should fix the issue) but I was unable to find an example that would work on my setup. I kept getting:
grub-install: error: cannot find EFI directory.
Finally after inspecting manpage for grub-install I did use additional switch:
sudo grub-install --efi-directory=/boot/efi /dev/nvme0n1
This did work and I was finally able to see GRUB welcome page and booted my PC just fine.