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 small bits of skin are 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 aromatics in the paste.
Pour the coconut milk slowly into the pan whilst stirring continuously, followed by the stock. Remove the skin of the chicken thighs and place the chicken thighs back into the pan. Simmer over a low heat for 15 minutes.
During this time, add the spaghetti to salted boiling water, setting your timer according to the packet.
After 15 minutes, add the pak choi and optional fish sauce. During this time, remove the chicken from the bone and cut into bite-sized pieces. Add it back into the pot. After 5 more minutes of cook time, you should be ready to serve up!
Serve up the Thai broth and chicken, then add a serving of noodles into the bowl. Top with some fresh coriander, sliced spring onion (the remaining greener end), chilli flakes and a squeeze of lime. Dig in!