Cook the rice noodles in boiling water according to the packet instructions, or until they are al dente, then drain and refresh in cold water. Drain again and snip into shorter lengths.
Using a teaspoon, scoop out the cucumber seeds along the middle of the half and discard. Thinly slice the cucumber on the diagonal and tip into a large salad bowl.
Add all the remaining salad ingredients, including the cooked noodles, season with salt and pepper, and toss together.
For the dressing, mix all the ingredients together in a small bowl. Just before serving, pour the dressing over the salad and toss together. Scatter with the peanuts, if using.
Instead of the crayfish, you could use prawns or pre-cooked meat such as sliced beef fillet or shredded chicken.
The vegetables, if covered and chilled, can be prepared up to 6 hours ahead. Add the noodles, crayfish and herbs and dress just before serving.
If you have no fresh mint, add a tablespoon of mint sauce instead.