Heat a frying pan with the oil, add the onion and garlic, and cook for 2 minutes, stirring frequently. Add the laksa or curry paste and fry for another 2 minutes, then stir in the coconut milk and stock. Bring to the boil, then simmer for 5 minutes.
While the sauce is cooking, put the noodles into a large heatproof bowl, pour over enough boiling water to cover and leave to soak for 5 minutes or according to the pack instructions. Separate the noodles gently using a fork or chopsticks.
Add the prawns to the sauce in the frying pan and cook, stirring frequently, for 3 minutes or until the prawns have turned pink. Stir in the green onions and bean sprouts. Squeeze the juice of one of the limes into the sauce. Taste and season. Quarter the other lime.
Drain the noodles and divide among four serving bowls. Ladle over the prawn laksa and serve immediately, garnished with lime wedges.
You can use fresh or frozen prawns for this recipe. There's no need to thaw them if frozen; cook for 4 minutes at step 3.