Step 1

Combine the stock, water and ginger in a large saucepan. Cover and bring to the boil over high heat. Stir in the fish sauce and lemon juice.

Step 2

Meanwhile, place noodles in a heatproof bowl, cover with boiling water and set aside for 3 minutes or until soft. Drain. Divide among serving bowls.

Step 3

Use a sharp knife to cut the beef across the grain into very thin slices. Arrange the beef slices and onion over the noodles.

Step 4

Remove the pan from the heat. Ladle the hot soup over the beef, onion and noodles (the hot soup cooks the beef). Top with the coriander leaves, bean sprouts and chillies. Serve immediately.