Preheat the oven to 190°C/Gas 7. In a small bowl, mix together the orange juice, coconut or maple syrup and fish sauce. Set aside. In a roasting tin big enough for the poussins, place the bashed lemongrass, lime leaves (if using), the flattened garlic cloves and ginger slices.
Spatchcock the poussins by placing them one at a time breast-side down on a chopping board, with the legs towards you. Using a good pair of kitchen scissors, cut up along each side of the backbone to remove it. Open the poussin out and turn over. With the heel of your hand, flatten the bird and spread out onto a high-sided baking tray that fits both birds snuggly.
Repeat with the other poussin. Save the backbone to make stock or add it to the roast for extra gravy flavour. Rub the poussins with vegetable oil and salt around all the grooves of the birds, along the insides of the thighs and wings. Wash your hands, then season the birds with black pepper. Place them on top of the lemongrass, garlic and ginger on the tray.
Around the tray scatter the whole halved garlic bulb, onions, lemon and orange pieces. Pour over the sauce mixture, rubbing it into the skin of the poussin, then place the star anise on the poussin.
Roast the poussin for 45–55 minutes or more until the juices run clear when pierced with a skewer, basting every 15 minutes. Rest the poussins for 10 minutes, then pour the juices into a small saucepan and bring to the boil. Add the cornflour mixture and whisk it in to avoid any lumps in the gravy, stirring well to thicken.
Garnish with chillies, citrus zest and spring onion. Serve with chips or steamed rice and plenty of vegetable sides.