Heat oven to 200°C
To make the curry sauce: Place the lemongrass, spring onions, garlic, ginger, chilli (if using), ground coriander, turmeric, sambal oelek, lime leaves, sugar and chicken stock in a blender and blend until smooth. Stir in the vegetable oil and 200ml (half) of the tinned coconut milk. Pour the curry sauce into the roasting tin.
Place the spatchcocked chicken on the sauce and sprinkle lightly with salt (the sauce is already salty). Do not add any oil or cover the chicken skin with sauce at this stage.
Roast for 45 - 50 minutes until chicken is cooked through and skin is crispy.
Remove tray from oven and transfer chicken to a large plate to rest.
Increase oven heat to 220°C. Pour remaining 200ml coconut milk into a roasting tray, stir to combine and return to oven for 10 minutes. The sauce will thicken and become glossy.
Dry roast cashew nuts until lightly browned. Chop into small chunks.
Return the whole chicken to the roasting tin along with any juices. Squeeze over the juice of 1 lime, sprinkle with roasted cashews, and scatter with coriander and mint. Serve in the tray for everyone to help themselves.