Using a speed peeler, peel the courgettes lengthways into long thin ribbons, then finely chop the chillies and add both to a bowl.
In a small jug, add the juice of the lemon and top up with double the amount of extra virgin olive oil.
Stir in the mustard powder and a pinch of sea salt, then mix well and pour over the salad. Toss to coat, then pick and scatter over the basil leaves and serve immediately.