- 1 lemongrass stalk
- 1 tablespoon Asian sesame oil
- 1 tablespoon vegetable oil
- 2 tablespoons minced shallot
- 2 small (1-inch) fresh Asian chiles such as bird or Thai, thinly sliced
- 2 tablespoons minced galangal or fresh ginger
- 5 cups low-sodium chicken broth
- 1 tablespoon Asian fish sauce
- 2 whole star anise
- 1 (3-inch) cinnamon stick
- 1 teaspoon salt
- 1/4 teaspoon freshly crushed white peppercorns
- 2 small tomatoes, each cut into 8 wedges
- 6 small scallions, thinly sliced
- 2 to 4 tablespoons fresh lime juice
- 1/2 pounds dried flat rice noodles
- 1 (1 pound) sirloin steak (about 1 inch thick)
- 3 (5 ounces) cups coarsely chopped spinach
- 1/4 cup coarsely chopped fresh mint
- Discard any dry outer leaves from lemongrass and coarsely chop bottom third of stalk. Heat oils in a 5-qt. heavy pot over moderate heat until hot but not smoking, then cook shallot and chiles, stirring, until dark brown (be careful not to burn), 3 to 4 minutes. Add lemongrass and galangal and cook, stirring, 1 minute.
- Add broth, fish sauce, star anise, cinnamon stick, salt, and white pepper and bring to a boil. Reduce heat and simmer 20 minutes. Pour through a fine sieve into another pot and discard solids. Add tomatoes, scallions, and lime juice and simmer until tomatoes are softened, about 10 minutes.
- Bring a large pot of water to a boil for noodles. Trim fat from steak and season with salt and pepper. Heat a well-seasoned ridged grill pan over moderately high heat until hot, then grill steak 4 to 5 minutes on each side for rare. Let stand 5 minutes before slicing very thin.
- Meanwhile, cook noodles in boiling water until tender, about 6 minutes, and drain. Divide among 4 large soup bowls and top with spinach and steak. Ladle soup over and sprinkle with mint.