Reinstall GRUB
E.g. after Windows installation:1
Boot from the Debian installer in Rescue Mode. The startup goes through a number of steps like the installation process, but eventually offers a menu option to install the boot loader to a partition.
There is no need to run chroot, grub-install, or update-grub manually.
Footnotes
NB: The Windows license is connected to the Microsoft account, so long in with that account when offered.↩︎