Example:

Boot to OMLx 'Live' with OMLx ISO (From my memory: In VirtualBox be sure the ISO is available and when you
see the ugly screeen navigate to 'Boot Manager' select that and select the option that says 'Boot to CDROM')

Mount your root partition to /mnt (example, change the exact path to path for your root partition):

1.   sudo mount /dev/nvme1n1p3 /mmt (example for nvme storage device)
     or: sudo mount /dev/sda2 /mnt (example for convential storage device)

2.   for i in /proc /sys /sys/firmware/efi/efivars /dev /dev/pts; do mount --bind $i /mnt$i; done

3.   chroot /mnt

4.   mount /boot/efi

5.   update-grub2

6.   grub2-install

7.   reboot