1. First, make the soup paste, either by mincing and stirring all paste ingredients together by hand, OR by placing the paste ingredients in a food processor and processing well. Set aside.
2. Place stock in a soup pot over medium-high heat. Add the orange juice, tamarind paste, sugar, fish sauce, plus the paste you just made. Stir well. When soup comes to a boil, reduce heat to medium.
3. Add the firmer of the vegetables first: the beans and the "whiter" parts of the Chinese cabbage, if using. Allow to simmer to 4-5 minutes, or until beans have softened.
4. Add all remaining vegetables, plus fish and shrimp. Simmer 2-3 more minutes, or until fish has turned firm and opaque-white and shrimp are pink and plump. Tip: Don't over-stir at this point, as this will cause the fish pieces to break up.
5. Remove the soup from the heat and do a taste-test. This soup should be spicy, salty, and sour but with overtones of sweetness. Adjust the taste by adding more fish sauce (instead of salt), then work on adjusting the sour-sweetness balance. Exactly how sour or sweet your soup tastes will depend on the sweetness of your oranges/orange juice, and the strength of your tamarind paste/lime juice (which is quite sour). Add 1 to 2 Tbsp. more brown sugar, as needed, OR up to 1/3 cup more orange juice (if juice is sweet). If soup is too spicy, add up to 1/2 cup coconut milk OR 1/2 cup more stock. If you'd like more spice and flavor, add a dollop or two of Thai chili sauce.
6. To serve, ladle soup into bowls. Add several half-slices of orange to each bowl, then sprinkle over the fresh coriander. (Note that cooked noodles or rice can also be added if you are serving this soup as the main entree). ENJOY!