In my system (Gentoo Linux), I boot via efistub and a built-in kernel command line that points to my root partition, so you don't even need Grub technically though I think it works basically the same way. Literally just 'root=PARTUUID=$whatever' compiled into the kernel, and you can set it as any valid rootfs target as far as I know. I don't even use an initramfs or initrd. I just boot right into my root partition in a couple of seconds.