sudo mount /dev/sdXXX /mnt
sudo mount /dev/sdXX /mnt/boot/efi
for i in /dev /dev/pts /proc /sys /run; do sudo mount -B $i /mnt$i; done
sudo chroot /mnt
grub-install /dev/sdX
update-grub

$ sudo chroot /mnt
# grub-install /dev/sdX
grub-install: error: /usr/lib/grub/i386-pc/modinfo.sh doesn't exist.
Please specify --target or --directory.

# ls /usr/lib/grub/
grub-mkconfig_lib  x86_64-efi  x86_64-efi-signed

# grub-install --target=x86_64-efi /dev/sdX
Installing for x86_64-efi platform.
Installation finished. No error reported.

sudo mount /dev/sdXXX /mnt
sudo mount /dev/sdXY /mnt/boot
sudo mount /dev/sdXX /mnt/boot/efi
for i in /dev /dev/pts /proc /sys /run; do sudo mount -B $i /mnt$i; done
sudo chroot /mnt
grub-install /dev/sdX
update-grub