In a large, wide, heavy-based pan at least 13cm deep, heat 5cm of vegetable oil over a medium-high heat (make sure the pan is no more than two-thirds full) until it reaches 190C. While the oil is heating, make the batter. Whisk together the cornflour, potato starch, flour and bicarbonate of soda in a bowl with a pinch of salt. Add the egg white, vegetable oil and 120ml water and whisk to form a thick batter.

Working in batches, coat the prawns in the batter, letting any excess drip back into the bowl. Carefully lower the prawns into the hot oil, one at a time, and fry, stirring occasionally, until golden brown, about 1 1/2 mins. Transfer to a wire rack or paper-lined plate to drain. Repeat with the remaining prawns, letting the oil return to 190C between batches.

Let the oil return to 190C, then carefully return the prawns to the oil and fry a second time for 1 1/2-2 mins until very crisp. Transfer to a wire rack or paper-lined plate to drain. Set aside.

To make the sauce, whisk together the sugar, tomato purée, soy sauce, vinegar, gochujang, sriracha and sesame oil in a bowl with the remaining cornflour and 60ml water until the sugar has dissolved. Set aside.

Heat the vegetable oil in a large frying pan over a medium-high heat and add the onion, garlic, red and green chillies. Cook for about 30 seconds, stirring, until just fragrant. Add the sauce mixture and cook for about 45 seconds, stirring, until the sauce thickens and becomes glossy. Add the fried prawns, toss quickly to coat and transfer to a platter. Scatter over the spring onions and serve immediately with rice.