Rinse lemon grass; cut off and discard tough tops and root ends. Peel off and discard coarse outer layers, then crush the tender inner part with the flat side of a large knife. Coarsely chop 1/4 cup cilantro leaves, reserving stems (save extra leaves for other uses).
In a 4- to 6-quart pan, combine broth, crushed lemon grass, cilantro stems, 1/4 cup green onions, ginger, chilies, garlic, and peppercorns. Bring to a simmer over high heat; cover, reduce heat to maintain a simmer, and cook 30 minutes. Pour through a strainer over a large bowl; discard solids.
Meanwhile, in another 4- to 6-quart pan over high heat, bring about 3 quarts water to a boil. Add noodles and cook, stirring occasionally, until tender to bite, 2 to 3 minutes. Drain.
Divide noodles between two large soup bowls and ladle broth over noodles. Sprinkle with chopped cilantro and remaining 1/4 cup green onions.