Heat half the oil in a wok or large fry pan over a high heat, and stir fry the beef in 2 batches tossing frequently for about 2 minutes or until just sealed.
Transfer to a bowl.
Reduce the heat to medium and add the remaining oil.
Add the onion and curry paste and cook for a further 2 minutes or until aromatic.
Add the capsicum, coconut milk and water.
Bring to the boil, then reduce the heat and simmer for about 3 minutes.
Add the bok choy and cook for about 2-3 minutes or until just tender.
Meanwhile, put the noodles in a heatproof bowl and cover with boiling water, allow to stand for 2 minutes to soften slightly.
Drain.
Add the beef, noodles, fish sauce, lime juice, sugar and spring onions to the curry.