1.For sweet soy sauce, combine stock, soy sauce, sake, sugar and coriander stems and roots in a saucepan and simmer over medium heat for 3 minutes, then strain and cool.
2.Combine potato flour and ½ cup water in a bowl to form a paste. Spread paste evenly over toothfish. Heat oil in a wok or frying pan over medium-high heat, add two pieces of fish and cook for 2-3 minutes each side or until cooked through. Transfer to an absorbent paper-lined plate and repeat with remaining fish.
3.Spoon sauce among bowls, top with fish and serve scattered with chilli and coriander leaves.
Note If Patagonian toothfish is unavailable substitute with snapper or hapuku fillets. Potato flour is available from health food stores and Asian supermarkets.
