In a small mixing bowl combine all the dressing ingredients and whisk well until the sugar dissolves.
If you would like the dressing on the mild side use 1 chilli and remove the seeds. Set aside.
For the salad
Place rice noodles in a large bowl and boil a full kettle. Pour boiling water over the rice noodles and mix well, submerging them. After about 5 minutes they should feel soft and tender. Rinse under cold water and drain.
In a large saucepan, bring a few cm of water to the boil. Add a steaming basket / stainless steel steaming insert / colander in a pinch. Add the prawns and steam for ± 2 minutes until pink and opaque. Remove the prawns straight away and spread out onto a tray to cool.
In a large salad bowl combine the spring onions, mint leaves, coriander, cucumber and carrot. Mix well.
Add the rice noodles and distribute them evenly making sure all the vegetables and herbs mix in well. Add the steamed prawns and mix.
Drizzle everything with a little dressing and toss to coat. Add the mushrooms and mix them in thoroughly, adding more dressing if necessary until the mushrooms are well marinated.
Serve the salad with plenty of lime cheeks for squeezing over and extra dressing on the side. Enjoy!