To spatchcock the chicken, put the bird on the worktop, breast-side down, with the legs pointing towards you. Using good kitchen scissors, cut through the flesh and bone along both sides of the backbone (and either side of the parson’s nose). Remove the backbone and keep it for stock (freeze until you’ve gathered a few). Turn over the chicken, then flatten it by pressing hard on the breastbone with the heel of your hand – you’ll feel it breaking and flattening under your hand. Remove any big globules of fat and neaten any ragged bits of skin.
To make the marinade, remove and discard the coarse outer layer of leaves and the green tips from the lemongrass. Chop the pale inner core finely, then pound using a pestle and mortar with the garlic, ginger and chillis. Transfer to a bowl, then add the rest of the ingredients. Put the chicken in a dish that will fit in your fridge. Gently loosen the skin on the breast, then work your fingers in between the skin and the flesh, including the legs. Spoon some of the marinade under the skin, then spread more marinade all over the chicken (use no more than half the marinade in total at this point). Cover the bird with cling film and chill in the fridge for a few hours, or overnight.
Heat oven to 180C/160C fan/gas 4. Put the chicken, breast-side up, in a roasting-tin in which it fits snugly (if there’s too much room around the bird, the marinade will burn). Scrape all the marinade from the dish over the bird. Season all over, then roast for 1 hr 10 mins. Baste after 15 mins with all the juices from the tin. Baste again after 30 mins, then cover with foil (if the chicken isn’t very brown, don’t cover with foil yet – keep checking until it is brown, then cover). After 50 mins, spoon over the remaining marinade, cover with foil again and cook for the final 20 mins.
To check that the chicken is cooked through, pierce the bird between the leg and the rest of the body – the juices should be clear. Cut into portions, then serve with rice and stir-fried green veg.