Whisk together the lime zest and juice, fish sauce, sugar and chillies until the sugar dissolves. Pour 1/3 of this mix into a bowl, add the oil and set aside (this will be the dressing). Put the remainder into a large food bag with 1 tsp salt, the garlic, turmeric and coriander stems. Slash each piece of chicken down to the bone a few times, then add to the marinade and squish it around well in the bag. Chill for at least 2 hrs, or ideally up to 24 hrs.
Heat oven to 180C/160C fan/gas 4 (if barbecuing, get the coals ready). Lift the chicken from the marinade and into a roasting tin. Roast for 30 mins until cooked through but not yet completely tender.
If you’re barbecuing, transfer the chicken from its tin to a hot barbecue for 10 mins more, turning once crisp (otherwise continue roasting in the oven). Spread the spring onions, cucumber and tomatoes over a platter, add the chicken, then finish with the herbs, avocados, nuts and dressing.
