Fixes: HP ignoring linux boot manager (Grub)

1. cd /boot/efi/EFI

Grub/       # or some custom name
Microsoft/  # Windows
Boot/       # default fallback (no OS)

2. Replace Windows boot efi with Grub efi.

sudo mv Microsoft Microsoft.bak
sudo mkdir -p Microsoft/Boot
sudo cp Grub/grubx64.efi Microsoft/Boot/bootmgfw.efi

3. custom boot entry for Windows to chainload from Grub.

Find UUID of /boot/efi: sudo blkid

(for example:- D064-C2CB)

edit sudo nano /etc/grub.d/40_custom:

menuentry "Windows Boot Manager (manual)" {
    insmod part_gpt
    insmod fat
    search --no-floppy --fs-uuid --set=root UUID_HERE  # <= UUID of /boot/efi
    chainloader /EFI/Microsoft.bak/Boot/bootmgfw.efi   # the folder we renamed in step 2.
}

4. Update grub (requires os-prober)

edit sudo nano /etc/default/grub: un-comment this line --> GRUB_DISABLE_OS_PROBER="false"

sudo os-prober
sudo grub-mkconfig -o /boot/grub/grub.cfg

Now you'll be able to boot directly into grub, from there you can decide to boot into Linux/Windows.