Photo: Vyacheslav Argenberg (CC BY 4.0) via Wikimedia Commons · source
Phở is all broth: beef bones simmered with charred onion and ginger and a few warm spices, then poured boiling over raw sliced beef so it cooks in the bowl.
2 kg (4 1/2 lb) beef marrow and knuckle bones, 500 g (1 lb) beef brisket in one piece
2 onions and 10 cm (4 in) ginger, unpeeled, halved
Spices: 4 star anise, 1 cinnamon stick, 4 cloves, 1 black cardamom, 1 tbsp coriander seeds
To season: 3 tbsp fish sauce, 1 tbsp rock sugar or sugar, 1 tbsp salt
600 g (1 1/4 lb) flat rice noodles (bánh phở), fresh or dried
250 g (8 oz) beef sirloin or eye of round, frozen 30 minutes and sliced paper thin
To serve: sliced scallions, cilantro, thinly sliced onion; in the south also bean sprouts, Thai basil, lime and chili
How to make it
Char (15 min): Char the onion and ginger directly over a gas flame or under the broiler until blackened in spots. Toast the spices in a dry pan 2 minutes until fragrant and tie them in cheesecloth.
Clean bones (15 min): Cover the bones with cold water, boil 10 minutes, drain and rinse them and the pot.
Simmer (4 hr): Return bones and brisket to the pot with 5 liters (5 quarts) water. Bring to a bare simmer and skim. Add onion and ginger. Take the brisket out after 1 1/2 hours, plunge it into cold water and chill. Add the spice bag for the last hour. Strain; season with fish sauce, sugar and salt. It should taste a little too salty on its own.
Noodles (5 min): Soak dried noodles in warm water 30 minutes, then blanch fresh or soaked noodles in boiling water 15 to 30 seconds; drain and divide among big bowls.
Build (5 min): Top the noodles with sliced cooked brisket and raw sirloin. Bring the broth to a hard boil and ladle it over so the raw beef turns pink. Scatter scallions, cilantro and onion; serve the herbs, lime and chili on the side.
Tips
Never let the broth boil hard while it simmers; it clouds.
Phở gà (chicken): a whole chicken, the same charred onion and ginger, 2 star anise only, 1 1/2 hours.