I found these very helpful
https://github.com/juan-vg/maas/wiki/Create-a-Debian-9-(Stretch)-image-that-works-with-UEFI-secure-booting
https://askubuntu.com/questions/882911/how-do-i-use-maas-to-create-a-debian-image
although I’m still struggling with debian 10 Debian10 fails on final reboot