Pat dry the fish with kitchen towels, sprinkle with salt and pepper and let sit for 10 minutes, then lightly dredge in cornflour.
Heat a large frying pan with 1 tablespoon oil over high heat. Stir-fry the fish until no longer opaque, 3-5 minutes, depending on size of your fish pieces. Set aside.
Heat the same frying pan with 1 tablespoon oil over high heat. Stir- fry the garlic until aromatic, 2 minutes. Add the onion and salt and cook until soft, 3 minutes. Mix in the water, tomato sauce, Worcestershire sauce, sugar, vinegar and salt.
Bring the mixture to the boil. Add the fish back into the pan and garnish with spring onions before serving.