Peel and devein the prawns. Put shelled prawns and all remaining shrimp ball ingredients in a food processor.
Cut tofu and bok choy into small pieces.
Put water, salt, soy sauce, mirin and pepper in a pot. Bring it to a boil on medium heat. When it starts boil, add bok choy and tofu.
Spoon shrimp balls into soup. When shrimp ball turns pink, skim off foam with a ladle. Mix 1 Tbsp starch with 2 tsp water and add the starch-and-water mixture to the soup.
Beat the egg. Pour the egg in a very thin stream over the surface. Turn off the heat. Add vinegar and mix gently.
You won't notice the vinegar, but a small amount of vinegar makes it taste better. Try it!