To prepare the chicken, place it breast side down on a chopping board. Use scissors to cut down either side of the backbone and remove it. Flip the chicken over and press down on the breastplate so that breast bones crack and the chicken sits quite flat. For the marinade, use a mortar and pestle (or a food processor) to pound the lemongrass, coriander roots and garlic to a fine paste. Stir through the soy sauce, fish sauce, dark sweet soy sauce and brown sugar. In a large bowl, combine the chicken with the marinade. Allow to marinate for 10 minutes (or up to an hour). Preheat the oven to 200°C/392°F. For the nuoc cham, whisk together the ingredients and set aside until ready to serve. Prepare a baking tray by lining it with foil and placing a baking rack on top. Place the chicken on the rack breast side up. Pour over the marinade. Pour 2 cups of water into the base of the baking tray. Roast for 40 minutes or until the chicken is cooked through (you can test by slicing into the leg joint and if the juices run clear, the chicken is cooked). Remove from the oven and allow to rest for 10 minutes. Cut the chicken into pieces and serve with cucumber, nuoc cham and steamed rice.