Heat the cooking oil in a pot and sauté the garlic, onion and tomatoes.
When the garlic, onion and tomatoes start to soften, add the miso. Cook for a few minutes until the vegetables liquefy some more and the mixture turns a bit pasty.
Pour in the tamarind juice. Season with patis to balance the sourness. If you think you don’t have enough liquid, or if the broth is much too sour, dilute with water.
Add the fish. Bring to the boil. Lower the heat cover and simmer for about ten minutes.
Add the mustard leaves and simmer for another three minutes.
Ladle the soup into your favorite soup tureen or serving bowl and serve at once.