Peel and clean the prawns. Heat 1 tbsp of the oil over a high heat in a non stick frying pan. Add the prawns and fry briskly on both sides until pink and cooked through. Add 2 tbsps lime juice then tip into a bowl and set aside.
Whisk together the remaining 2 tbsps of lime juice with 6 tbsps of extra virgin olive oil. Season to taste with the fresh chillies, salt and freshly ground black pepper. Mix half of the dressing into the prawns.
Peel each mango and cut the flesh away from the stone in four segments. Finely slice each segment lengthways. Place in a bowl with the red onion, salad and coriander leaves.
Halve, stone and peel the avocados then slice their flesh lengthways. Dress in the remaining dressing then mix into the salad with the prawns. Divide between plates, garnish with the lime halves and serve immediately.