To make the sauce, slice the white part of the spring onions lengthways into quarters. Cut the quarters in half across the middle. Cut the green parts lengthways into strands and set aside.
Heat the oil in a large frying pan or wok until very hot.
Add the white part of the onions, green pepper, garlic and ginger. Stir-fry for 30 seconds.
Reduce the heat and stir in the soy sauce, vinegar, sugar, tomato
ketchup and fish sauce and 100ml cold water. Cook for a few seconds, stirring, until the sauce is well combined then add the pineapple and chilli.
Cook for 3-4 minutes until the pineapple is softened and the sauce is hot.
Mix the cornflour with 1 tablespoon cold water and stir into the sauce. Cook until thickened and glossy and season with plenty of ground black pepper.
Cut each fish fillet into 3 pieces and toss with the cornflour and salt on a plate until evenly coated. Pour the oil into a small non-stick frying pan or wok and put over a high heat.
When the oil is hot, drop the floured fish pieces into the pan using tongs and regulate the heat so as not to burn them. Fry hard for about 2 minutes on each side until golden and crisp. Drain on kitchen paper.
Do this in batches and keep the cooked fish in a warm oven.
Reheat the sweet and sour sauce until bubbling then spoon into warmed dishes and place the hot fish on top. Scatter the chopped green spring onion on top. Serve immediately with boiled or egg-fried white rice.