Put all the dressing ingredients in a jar and shake together.
Fry the garlic in the oil in a heavy-based saucepan over a medium heat until golden and crispy. Remove from the oil with a slotted spoon and set aside to cool.
In the same oil, stir-fry the prawns with the roasted cashew nuts for about 2-3 mins until the prawns are just pink. Add to the garlic, spoon over 2 tbsp of the dressing and set aside to cool.
When ready to serve, peel, stone and dice the avocado, and toss together with the cabbage, mango, herbs, spring onions and prawn mixture. Serve immediately, handing round the remaining dressing in a separate bowl.
Make the dressing up to 1 week ahead and keep in a jar in the fridge. Cook the prawn mixture the day before needed. Keep chilled and toss everything together just before serving.
This dressing is also delicious spooned over pan-fried chicken fillets and served with basmati rice.