Place the chicken breasts and stock in a large saucepan. Bring it to a boil, then reduce to a simmer for about 15 minutes until the chicken is cooked through. Remove the chicken with a slotted spoon, then chop into pieces and set aside.
Add the shallots, garlic, ginger, chilli, curry paste, fish sauce, sugar and mushrooms to the stock and simmer for 5 minutes over medium heat.
Next add in the pak choi, coriander and cooked chicken to the pan and simmer for 3 more minutes. Stir in lime juice and season with salt and pepper.
Boil a separate pot of water and cook the noodles per the package instructions. Drain, and divide the noodles between soup bowls. Ladle the soup over top, and serve.
