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Heat the oil in a wok or deep frying pan. Toss in the onion, squeeze in the coriander purée and sprinkle over the crushed chillies to taste (go easy at this stage). Stir fry for 5 minutes until the onion is softened but not browned.
Pour in the tomatoes and 11⁄2 canfuls of hot water, add the tomato purée and sprinkle over the bouillon powder. Season well. Bring to a bubble, stirring, then lower the heat a little and let the sauce simmer for about 15 minutes until slightly reduced but still sloppy. While the sauce is simmering, break the noodles into a bowl and pour in enough boiling water to cover. Stir and set aside.
When the sauce is ready, drain the noodles and tip them into the sauce with the frozen prawns. Stir well and heat through for 2 minutes only – just to defrost and heat the prawns. Taste for seasoning and add more chilli flakes and a little sugar if you like. Serve in bowls with spoons and pass round chunky hot bread for scooping up the last of the sauce.