Put the yogurt in a fine sieve and leave to drain for 30 minutes.
Peel the carrots, halve and cut into very fine strips.
Peel the onions and cut into very fine strips.
Heat the oil in a pan. Add the mustard seeds and toast briefly until they start to pop.
Add the carrots and onions to the pan. Add salt, cumin and chilli powder and cook over a medium heat for 3-4 minutes until tender but not soft.
Remove the mixture from the pan and allow to cool.
Wash the mint, shake dry and chop the leaves. Juice the lemon. Mix the drained yogurt with the mint and some lemon juice.
Mix the carrots with the yogurt sauce and serve immediately.