Booting via GRUB 2 prompt

Did your update-initramfs not complete before you rebooted your zfs-on-root Ubuntu install? Here's how to boot it from the GRUB prompt using a previous kernel, assuming that your boot filesystem is on (hd0,gpt3), the zfs dataset name of your boot filesystem is bpool/BOOT/ubuntu/HEAD, the zfs dataset name of your root filesystem is rpool/ROOT/ubuntu/HEAD, and you're wanting to boot version 5.4.0-42-generic of the kernel:

grub> insmod zfs
grub> set root=(hd0,gpt3)
grub> linux /BOOT/ubuntu/HEAD/@/vmlinuz-5.4.0-42-generic boot=zfs zfs_force=1 root=ZFS=rpool/ROOT/ubuntu/HEAD net.ifnames=0 biosdevname=0
grub> initrd /BOOT/ubuntu/HEAD/@/initrd.img-5.4.0-42-generic
grub> boot
...
... kernel messages here ...
...
(initramfs) /sbin/modprobe zfs
(initramfs) zpool import rpool -R /root
(initramfs) exit

I also add:

net.ifnames=0 biosdevname=0

to ensure that all Ethernet device names use the naming scheme ethX for consistency. That part is not necessary.