Combine cumin, vinegar, olive oil, garlic, red pepper flakes, cinnamon, brown sugar, chilli powder and tomatoes; whisk to combine.
Put the goat shanks in a large bowl and add 2/3 of the marinade; coating on all sides.
Preheat the oven to 170 degrees C. Season the shanks with salt and pepper, transfer to the oven and braise until very tender, about 2 hours. (Flip the lamb shanks at 1 hour).
Remove from the oven and let cool slightly. Skim off any excess fat and spoon any reduced marinade from the pan into a small mixing bowl and mix with the reserved (uncooked) marinade.
Remove the shanks and pull the meat off the bone, discarding the bones and fat.
Shred the meat with your fingers (or forks) and transfer to a bowl. Pour the warmed braising liquid over the meat and toss to coat.
In a dry pan, brown tortillas on both sides.
Divide the goat among corn tortillas and top with coriander and onions.