1. Place the paste, onion, chilli and garlic in a heavy-based pan with a little spray oil and cook over medium heat, stirring constantly for 2 minutes.
2. Add the evaporated milk, coconut essence and stock and stir to combine. Bring to the boil over a high heat. Reduce heat to medium-low and simmer, partially covered, for 5 minutes.
3. Add the prawns to the soup and cook for 1 to 2 minutes or until they curl and change colour. Add the bean sprouts and cook for a further minute.
4. Meanwhile, add the rice noodles to a medium saucepan of boiling water and cook for 3 minutes or until tender. Drain and divide among four bowls.
5. Place the coriander and spring onions into serving bowls with the noodles and ladle the hot soup over the top. Serve immediately.