Briefly whisk together the egg white, cornflour, and seasoning until smooth. Add the prawns and stir to coat.
Heat the groundnut oil in a large wok over a moderate heat and stir-fry the prawns for 1 minute. Remove and drain on kitchen paper. Clean the wok, then heat the groundnut oil for the sauce until hot over a moderate heat.
Stir-fry the onion, red chilli, pepper, and carrot for 2-3 minutes, then add the water chestnuts and cook for a further minute. Whisk together the sherry vinegar, tomato ketchup, soy sauce and sugar with 50ml of warm water. Pour into the wok, then add the prawns and bring to a simmer.
If desired, thicken the sauce with the cornflour paste; whisk it in and bring to a simmer to thicken. Spoon onto a serving platter and garnish with a sprig of coriander before serving.
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