Bring beef stock, water, star anise, cinnamon stick, and sliced onion to a boil in a pot.

Reduce heat and simmer for 15 minutes.

Add fish sauce and taste. Adjust seasoning as needed.

Blanch fresh flat rice noodles in the simmering broth for about 20 seconds. If using dried noodles, cook according to package instructions in a separate pot.

Remove noodles from broth and divide into serving bowls.

Blanch sliced beef in the broth for 10 seconds, or until desired doneness. Remove and place on top of noodles.

Pour generous amount of broth over beef and noodles.

Garnish with chopped spring onions and sliced chili padi (optional).

Serve with a side of bean sprouts and fresh herbs.