GRUB and Initramfs

First retrieve the UUID for /dev/nvme0n1p3:

blkid -o value -s UUID /dev/nvme0n1p3

The UUID will be used for GRUB

Configuring GRUB

With the UUID retrieved, push to /etc/default/grub:

blkid -o value -s UUID /dev/nvme0n1p3 >> /etc/default/grub

Inside /etc/default/grub, add the following parameters:

# /etc/default/grub
...
GRUB_CMDLINE_LINUX_DEFAULT="... rd.luks.name=<UUID>=encrypted root=/dev/mapper/encrypted"
...

Do not remove nor replace existing parameters

Uncomment CRYPTODISK:

...
GRUB_ENABLE_CRYPTODISK=y
...

If you have not removed the UUID entry at the bottom of /etc/default/grub, remove it to avoid any errors

grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=GRUB
grub-mkconfig -o /boot/grub/grub.cfg

Open /etc/mkinitcpio.conf to make the following changes:

MODULES=(btrfs)

HOOKS=(... block sd-encrypt filesystems ...)

Regenerate the initramfs:

mkinitcpio -P