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Mix the soy sauce, vinegar, ketchup, nam pla and 100ml cold water. Heat the tbsp sunflower oil in a large frying pan or wok until very hot. Add the onion, garlic and ginger. Stir-fry for 30 secs, then reduce the heat and stir in the sauce.
Add the pineapple and chilli, then cook for 3-4 mins until the pineapple is slightly soft and the sauce is hot. Mix the cornflour with 1 tbsp cold water and stir into the sauce. Cook until thickened and glossy. Season with plenty of ground black pepper.
Toss the fish with the flour and a pinch of salt until evenly coated. Place a medium saucepan over a high heat and add enough oil to rise 1cm up the sides of the pan. When the oil is hot, lower the fish into the pan using tongs and fry hard for about 1-2 mins on each side until golden and crisp. Drain on kitchen paper. Do not allow the oil to overheat and do not leave the hot oil unattended.
Reheat the sauce until bubbling, then spoon into warmed dishes and place the hot fish on top. Scatter with shredded spring onion and serve immediately.