# /dev/sdb is the the usb device with the ubuntu installation. Encryped and lvm
udiskctl unlock -b /dev/sdb3 # To decrypt
mount /dev/mapper/ubuntu--vg-root /mnt
for i in /sys /proc /run /dev; do sudo mount --bind "$i" "/mnt$i"; done
mount /dev/sdb2 /mnt/boot     # sdb2 is the boot partition
mount /dev/sdb1 /mnt/boot/efi # sdb1 is the "EFI System"
chroot /mnt
grub-install --removable /dev/sdb
update-grub