Step 1

Cut the prawns in half lengthways, along the back, removing the black vein running through the centre. Give them a quick wash, pat dry with kitchen paper and chop into pieces. Put onto a plate, sprinkle with 1 tsp salt, and transfer to the fridge while you make the chilli water.

Step 2

Bash the chillies with a pestle in a mortar until you make a vibrant green paste. Add a little coarse salt to help break the skins down if you need to (or you can whizz in a small blender). Stir in the lime juice and garlic. Scrape into a bowl and season generously. Toss in the prawns, so they are completely coated, and chill again for 30-40 minutes while you prep the other ingredients. Stir after about 10 minutes so the prawns marinate evenly.

Step 3

Check the prawns, they should be pink, and opaque. Keep marinating if they still have a little greyness. Add the onion, cucumber and avocado to the prawns, toss lightly, and then tip onto a platter or plates. Scatter over the coriander, and serve with tortilla chips.