Check installed partitions

These are some helpful commands to check partitions

fdisk -l
parted -l
lsblk -o PATH,SIZE,RO,TYPE,MOUNTPOINT,UUID,MODEL
blkid
hwinfo --block --short

Boot and get inside installed linux

sudo mount /dev/sda2 /mnt
sudo mount /dev/sda1 /mnt/boot/efi
for i in /dev /dev/pts /proc /sys; do sudo mount -B $i /mnt$i; done
sudo chroot /mnt

You can update the grub as follows

After doing chroot you can update grub of the installed linux as follows

apt install grub-efi-amd64-signed shim-signed 
grub-install /dev/sda --uefi-secure-boot
update-grub
exit

Check the UEFI entries

efibootmgr [--verbose]