First, make the dressing. Put the shallot, chillies and lime juice in a small bowl. Season, then whisk in the oil. Add the honey to taste, if using. Heat the oven to 200C/180C fan/gas 6.
Stand one of the corn cobs upright in a dish, then, holding it at the top, slice down the sides using a very sharp knife to remove the kernels. Work round the cob until all the kernels have been removed. Repeat with the remaining cobs, then put the kernels in a small roasting tin and the peppers in a second tin. Season both, then add 1½ tbsp oil to each. Toss both to coat. Roast the peppers on the top shelf of the oven until slightly charred, about 30 mins. Roast the corn, tossing halfway through, for 15-20 mins.
Tip the corn into a bowl and set aside. Brush the spring onions with the extra oil, put in the roasting tin and roast for 12-15 mins, until soft and slightly charred.
Chop the avocado flesh or slice it. Immediately pour over half the lime juice to prevent it browning, then season.
Slice the roasted peppers into strips, then arrange them, the spring onions and corn in a broad, shallow bowl. Add the coriander, treating it like a salad leaf (pull off any very coarse or long stalks). Spoon over the dressing.
Pat the prawns dry with kitchen paper – if they’re wet, they won’t fry properly. Heat the remaining 1 tbsp oil in a large frying pan and, when really hot, add the prawns. Cook until they turn from grey to pink – this will happen very quickly. When you see this starting to happen, add the spices. Toss to combine, then season and squeeze over the juice of the remaining lime. Tip the prawns onto the salad and serve immediately.