I note that my situation may not be unique.
Recent Lenovo laptop, came with W10.
I have added a 1 TB WD Green ssd to complement the UMIS 256 GB NVMe.
Added, and removed LinuxMint as SECURE BOOT.
Added and removed PClinuxOS as LEGACY BOOT.
Installed Leap 15.3 with SECURE BOOT ENABLED.
I have made several attempts to install a BOOT manager to achieve DUAL booting,
I have only achieved DUEL booting, it will boot from the USB I installed from.
Execution of command “”/usr/sbin/shim-install", “–config-file=/boot/grub2/grub.cfg”]]" failed.
Exit code: 1
Error output: No valid EFI partition
I change to REMOVE MBR as seen in another post/answer.