Combine the sugar, fish sauce, lime juice and oyster sauce in a small mixing bowl. Whisk until smooth and set to one side. Heat the sesame oil in a wok over a medium-high heat and stir-fry the onion and peppers for 2-3 minutes until they start to soften.
Add the chilli and prawns to the wok and continue to stir-fry for 3-4 minutes until the prawns turn pink and tender. Add the prepared sauce and toss well.
Remove from the heat and stir in the cucumber and coriander. Adjust the seasoning with soy sauce if necessary. Arrange the noodles in serving bowls and spoon the prawn stir-fry on top. Serve immediately.
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