Wrap the steak in plastic wrap and freeze for 40 minutes—this will make it easier to slice. Meanwhile, put the brown onion, fish sauce, star anise, cinnamon stick, pepper, stock, and 2 cups water in a large saucepan. Bring to a boil, then reduce the heat, cover, and simmer for 20 minutes. Discard the onion, star anise, and cinnamon.

Cover the noodles with boiling water and gently separate. Drain and refresh under cold water. Thinly slice the meat across the grain. Divide the noodles and scallion among four deep bowls. Top with the beef, mint, bean sprouts, onion, and chili. Ladle the hot broth over the top and serve with the lemon wedges.