Heat oil in a large pot or dutch oven over medium low heat. When oil is warm, add minced ginger, lemongrass and curry paste. Stir constantly and be careful not to burn the curry paste. Stir until fragrant, about 1 minute.

Add chicken broth, fish sauce, brown sugar and mushroom (save the golden needle mushroom to add later, because they cook more quickly). Simmer covered for 15 minutes, until mushrooms turn soft.

Add coconut milk and mix well with a ladle. Taste the soup and adjust seasoning if necessary (be careful of the hot soup).

Carefully add fish balls one by one into the soup. Simmer covered for 5 minutes.

(Optional) If you want to add some green beans or golden needle mushrooms, add them into the soup with the fish balls and simmer together, until the vegetables are cooked through.

Turn off heat. Add lime juice and stir well.

Serve warm.