2) Preheat the oven to 180C/Gas 4. In a baking dish, toss together the tomatillos, onion, jalapeno and garlic. Toss with extra-virgin olive oil and season, to taste, with salt and pepper.
3) Roast until tender and caramelized, about 20 minutes. Remove from the pan and cool for about 5 minutes then add to a food processor.
4) Add the avocado, lime juice and coriander and blend until smooth. Season, to taste. Pour into a large bowl and chill.
5) Meanwhile, preheat the griddle to medium-high. Remove the prawns from the bowl and brush with oil. Place on the griddle and cook until opaque.
6) Remove from the heat to a platter and serve with the tomatillo sauce. Garnish with lime zest, diced tomato, and chopped coriander.