Original from https://ubuntuforums.org/showthread.php?t=2264947 which works for me
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sudo su
apt-get install lvm2
vgchange -a y
(do any lvm management you need here, I didn't need any.)
mkdir /mnt/system
mount /dev/mapper/ubuntu--vg-root /mnt/system
mount /dev/sda1 /mnt/system/boot
for i in /dev/pts /dev /proc /sys; do mount -B $i /mnt/system$i; done
chroot /mnt/system
update-initramfs -k all -c
update-grub
exit
for i in /dev/pts /dev /proc /sys; do umount /mnt/system$i; done
umount /mnt/system/boot
umount /mnt/system
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