Preparation 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.
Preparation 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.