Roughly chop the coriander stalks (and roots, if you have them) and mix with the ginger, garlic, lemongrass, chillies and sugar. Finely chop in a food processor, or pound to a paste in a large pestle and mortar.
Add the sesame oil, two tablespoons of fish sauce, and the lime zest and juice, and whizz or pound again.
Cut three slashes on each side of the mackerel, then rub the marinade all over, including inside the cavity. Leave to marinate while you prepare the rice. The fish can be left to marinate for up to two hours.
Put the rice, coconut milk and remaining two tablespoons of fish sauce in a saucepan and add cold water to come 3cm above the rice. Bring to a boil, cover with a tight-fitting lid and cook on a high heat for five minutes.
Turn down the heat to its lowest setting, cook for eight minutes more, then take the pan off the heat and leave, covered, in a warm spot for 10 minutes.
While the rice is cooking, heat the oven to 220C/425F/gas mark 7. Put the mackerel on a large, foil-covered baking tray, spoon over any excess marinade, and roast for 12-15 minutes, until the flesh comes away easily from the bone.
Run a fork through the rice, and serve alongside the mackerel with wedges of lime.