1.Combine diced ling fillets, eggwhite, cornflour and sesame oil in a food processor and season to taste. Pulse until a coarse paste forms. Using wet hands, form tablespoon-sized balls of fish mixture.
2.Heat 6 cups of water, ginger, shiitake mushrooms, soy sauce and one-third of green onion in a saucepan on medium, until simmering.
3.Drop fish balls into simmering broth and cook for 2-3 minutes, until fish is cooked through. Remove fish balls from broth.
4.Strain broth into serving bowls. Add pak choy and fish balls. Stir through coriander leaves and serve topped with remaining green onion.