Place the cucumber in a large bowl, sprinkle over the lemon juice and season with salt. Stir to mix well and set aside.
Heat the oil in a small frying pan over a medium heat. Add the mustard seeds, curry leaves and chilli, and stir-fry for 1–2 minutes until fragrant and the mustard seeds start to 'pop'.
Add the contents of the pan to the cucumber mixture. Toss to mix well, sprinkle over the chopped peanuts and serve immediately.