Crush lemon grass slightly. Cut stalk into about 3-inch lengths.
Slice chili crosswise into thin rings; discard stem.
In a 3- to 4-quart pan, combine lemon grass, chili rings, galangal, lime leaves, and broth. Bring to a boil over high heat, cover, and simmer 15 to 20 minutes.
Meanwhile, in a 6- to 8-quart pan, bring about 3 quarts water to a boil over high heat. Add noodles and cook, stirring occasionally, until barely tender to bite, 3 to 4 minutes. Drain well. Place noodles in 5 or 6 large soup bowls.
With a slotted spoon, scoop lemon grass, chili rings, galangal, and lime leaves from broth and discard.
Stir shrimp into broth and cook until they are opaque in thickest part (cut to test), 2 to 3 minutes. Transfer shrimp with a spoon onto noodles in bowls.
To broth, add lime juice and fish sauce to taste. Ladle into bowls. Sprinkle with cilantro and onions. Add chili oil to taste.