Thanks for the above markc.

In my case, I had multiple disks (zfs raid 10). This makes it a little easier since you need to remove old kernels from all of the disks:

mkdir /tmp/fixme;
for i in $(cat /etc/kernel/proxmox-boot-uuids); do
    echo $i;
    mkdir /tmp/fixme/$i
    mount /dev/disk/by-uuid/$i /tmp/fixme/$i
done

Then df should show one of the UEFI partitions is full.

Poke around a bit a look at the directories that were mounted here. You should find some old kernels.

Then you can remove them with something like:

find /tmp/fixme/ -type f -iname "*5.4*" -exec rm {} \;