Cut the backbone from the chicken. Press down firmly on the breast bone to flatten the chicken. Rub with oil, salt, turmeric, black pepper and cumin. Place on a rack over a baking tray and tuck in the wings.
Roast at 200°C for 50 minutes- 1 hour. Grill for extra colour if necessary.
Meanwhile, add all of the paste ingredients to the blender. Whizz until smooth, then fry in 2 tbsp oil over medium heat until the oil separates from the masala. Add one tin of coconut milk, a pinch of salt and the juice of 1 lime. Cook for a further 5 minutes, then remove from the heat.
Add sauce to a large serving platter. Joint the chicken and place atop the sauce. Garnish with fried onions.