Place the chicken breasts and stock in a large saucepan. Bring up to the boil, then reduce the heat and simmer very gently 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 poaching stock and simmer for 5 minutes over a medium heat.
Add the pak choi, coriander and cooked chicken to the pan and simmer for a final 3 minutes. Stir in the lime juice and season to taste with salt and freshly ground black pepper.
Just before serving, cook the noodles in a pan of boiling water according to the packet instructions. Drain and divide the noodles between six soup bowls. Ladle the hot soup over the top to serve.
