Place the stock, ginger, chilli and spring onions in a large saucepan and bring to the boil. Meanwhile, heat a griddle pan until very hot and brush the steak with the oil. Griddle the steak for 2 mins each side for medium-rare. Transfer to a chopping board and leave to rest for 1 min, then thinly slice.
Add the pak choi to the stock, then the noodles, bring to the boil and simmer for 3 mins until tender. Ladle the stock, noodles and pak choi into large serving bowls and top with thin slices of steak.