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Tip the prawns in a bowl and mix with the soy, then set aside for a few minutes. Stir in the cornflour.
Heat the oil in a wok over a high heat to a depth of 3-4cm. When it’s shimmering or a cooking thermometer reads 170C, fry the prawns in batches for about 2 mins until crisp and golden, turning them with a slotted spoon halfway through. Carefully lift onto a plate using the slotted spoon. Turn off the heat. Leave the pan for a few minutes to cool down.
Carefully pour all but 1 tbsp oil from the wok – keep the remainder for use in another recipe or discard.
Put the wok back on a high heat. When the oil is shimmering, carefully add the onion and peppers, and stir-fry for 1 min. Add the garlic and ginger, and continue to cook for 30 seconds, then scatter over the sugar and pour in the vinegar. Turn up the heat and boil for 2 mins. Stir in the soy and ketchup to make a thick, glossy sauce. When you’re ready to serve, tip the prawns back into the pan and stir to coat in the sauce. Scatter over the chopped coriander and serve with cooked rice on the side.