Heat the sesame oil in a large, deep saucepan over a medium heat and fry the mushrooms for 3-4 mins until evenly coloured. Add the garlic and chilli flakes, and cook for another minute.

Add the stock (or crumble in the stock cube and add 400ml water), and bring to the boil. Tip in the noodles and pak choi, reduce the heat and simmer for 3-4 mins until the noodles are warmed through. Ladle the soup into a bowl and season with a splash of soy sauce, squeeze of lime juice and the crispy chilli in oil. Serve straightaway.