Assumptions:
– fs / is on /dev/md2
– fs boot is on /dev/md1
– you are using 2 harddisks (sda, sdb)
– you are using grub2
– grub2 is nor working (fe: error: symbol ‘grub_calloc’ not found.)

First boot your system with a rescue-disk/livecd. Then:

# mount /dev/md2 /mnt
# mount /dev/md1 /mnt/boot
mount --bind /proc /mnt/proc/ ; mount --bind /dev /mnt/dev/ ; mount --bind /sys /mnt/sys/
# chroot /mnt
# grub2-install /dev/sda   # or /dev/nvme1n1 - depends on your disks :)
# grub2-install /dev/sdb

you should see “Installing for i386-pc platform. Installation finished. No error reported.”

reboot.