Restore GRUB to EFI partition

Boot from USB Installer then

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  

while

  • sdXXX is root partition
  • sdXX is EFI partition
  • sdX is a boot disk