Start by putting the sugar in a small mixing bowl and add the fish sauce, lime juice and oyster sauce. Whisk it until it's blended and smooth.
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. Now you can conveniently add the chopped chilli and choi sum leaves and stir-fry for 2 mins.
In a wok, heat some oil and fry some prawns for two minutes until they are hot. Add the prepared sauce and toss well to combine.
Remove the pan from the heat and stir through the coriander seeds. Cook the noodles to pack instructions, then serve them on their own or tossed with the prawn stir-fry if preferred.