Combine the sugar, fish sauce, lime juice and oyster sauce in a small mixing bowl. Whisk until smooth and set aside.
Heat the oil in a wok or large frying pan over a medium-high heat, then add the onion, garlic and pepper and stir-fry for 2-3 mins until they start to soften. Add the chilli and choi sum and stir-fry for 2 mins.
Add the prawns to the wok and stir-fry for 2 mins until hot through. Add the prepared sauce and toss well to combine.
Remove from the heat and stir in the coriander. Heat the noodles to pack instructions. Arrange the noodles in serving bowls and spoon the prawn stir-fry on top, or toss together and serve.