To update the grub menu automatically after installing or removing a kernel, write the following lines to file /etc/pacman.d/hooks/grub-auto-update.hook:
[Trigger] Operation = Install Operation = Remove Type = Package Target = linux Target = linux-lts [Action] Description = Update grub after installing or removing a kernel. When = PostTransaction Depends = grub Exec = /bin/sh -c "/usr/bin/grub-mkconfig -o /boot/grub/grub.cfg"
Note that you may need to create the directory first:
sudo mkdir -p /etc/pacman.d/hooks