Boil the water in a large soup pot. Add the tamarind. Cover and continue to boil for 30 minutes. Remove the tamarind from the water using a kitchen strainer. Let the tamarind cool down and put it on a sturdy sieve. Position the sieve over the pot and then extract the tamarind juice out by placing a spoon on top of the tamarind. Push it against the sieve. Discard the pulp. The water should still be boiling at this point.
Add tomatoes, onion, and miso paste. Cover and boil for 15 minutes.
Gently slide-in the tuna jaw slices. Cook for 20 minutes.
Add the peppers and daikon radish. Cover and continue to cook for another 10 minutes.
Add the mustard greens and pour the fish sauce on the pot. Stir. Cook for 8 minutes.
Transfer to a serving bowl. Serve with rice along with a small bowl of fish sauce and red pepper flakes on the side.