Pour the stock into a Bessemer Chinese Wok, add shallots, ginger, star anise, lemon juice, fish sauce, chillies and sugar. Bring to the boil, reduce heat and simmer gently for about 20 minutes.
Add onion and simmer a further 5 minutes. Add sliced beef and simmer for 2-3 minutes. Remove from heat and discard star anise.
Dip the rice noodles in boiling water for about 1 minute, then drain and divide between 4 large bowls.
Spoon over sliced meat and onion and pour over stock. Divide lime juice between bowls and top each with fresh basil and mint leaves and bean sprouts. Serve with wedges of lime and chilli sauce.