Boil a pot of water and blanch the noodles and bok choy lightly. Drain and divide among 4 serving bowls.
Place all the ingredients for the fish balls into a food processor and process into a paste. Form into small balls and set aside.
Place all the ingredients for the spice paste into a clean blender and process until fine. Set aside.
Place all the ingredients for the chilli soy sauce in a clean blender and process until fine. Set aside.
Heat the oil in a large pot over medium heat. Add the blended spice paste and fry until fragrant.
Add the water and bring to a boil. Add the fish stock cube and salt. Turn down the heat and let it simmer for 10 minutes.
Add the fish balls and stir gently. The fish balls are done when they float.
Ladle the hot soup with some fish balls into each serving bowl with the noodles and bok choy. Garnish with chillies, coriander leaves and crisp-fried shallots. Add chilli soy sauce to taste. Serve immediately.