1. Heat a cooking pot and then pour-in cooking oil.
2. Put the onions in when the oil becomes hot, and then saute for 2 minutes.
3. Add the miso and cook for another 2 minutes.
4. Put-in the fish sauce and water, and then stir. Bring to a boil.
5. Add the tomatoes and tamarind soup mix. Cook for 5 minutes.
6. Put-in the long green chilis and milk fish. Cook for 15 to 18 minutes in low heat.
7. Finally, add the mustard leaves and turn off the heat. Cover the pot and let the residual heat cook the greens.
8. Transfer to a serving bowl, and then serve.
9. Share and enjoy!