Add the olive oil to a large saucepan on a low heat. Place the chicken thighs skin-side facing down and leave to gently fry for around 6 minutes, gradually increasing the heat. This will brown the skin (making it very easy to remove) and flavour the oil.
Remove the chicken from the pan, don’t worry if some of the skin is stuck to the pan, this will add to the flavour of the soup. Add half of the chopped spring onions (whiter end) to the pan to fry for a couple of minutes before adding the red Thai paste, peanut butter, tomato puree and a splash of water to combine. Gently fry for a couple of minutes to bloom the spices.
Pour the coconut milk in gradually, whilst stirring, followed by the stock. Remove the skin and then add the chicken thighs into the soup pan. Simmer for 25 minutes.
After 25 minutes, add the pak choi and optional fish sauce. Also add the spaghetti at this stage to both cook for a further 10 minutes. During this time, remove the chicken from the bone and cut into bite-sized pieces.
Serve up with some fresh coriander, sliced spring onion, chilli flakes and a squeeze of lime.