Sit the chicken in a flameproof roasting tin or shallow casserole dish and stuff the lemongrass, lime halves and ginger in the cavity. Mix half the curry paste with the oil and use your hands to smear the curry paste all over the chicken and under the skin. Can be prepared up to a day ahead and left to marinate, covered, in the fridge. Take out an hour before roasting.
Heat oven to 200C/180C fan/gas 6. Cover the tin with foil and roast for 40 mins, then remove the foil and roast for another 30 mins until the chicken is cooked through and golden. Lift the chicken, draining the roasting juices back into the tin, and sit on a board or dish that will catch the juices. Use a pair of tongs to remove the lime, lemongrass and ginger from the cavity and place them in the tin with the remaining curry paste and brown sugar.
Place the tin back on the heat and sizzle everything together, squashing the lime with the back of a spoon until everything is sticky. Pour in the coconut milk and bring to a simmer, stirring well, then add any resting juices and the fish sauce. Simmer again briefly, then sit the chicken back in the tin and scatter with coriander, if using, and the chilli. Bring the chicken to the table and serve with rice and greens with the curry sauce spooned over, and extra lime wedges on the side for squeezing over.