Blacken ginger and onions by threading on a metal skewer and holding over a gas burner flame and turning until scorched on all sides (1-2 minutes).

Leave one piece ginger whole, chop the other when cool.

Combine broth, chicken, fish sauce, peppercorns, blackened onions, and the whole ginger piece in a large saucepan and bring to a boil.

Reduce heat and simmer, covered, until the chicken is cooked through, about 5 minutes.

Remove chicken and shred into bite-sized pieces.

Continue simmering broth for about 20 minutes.

In the mean time, soak vermicelli in a large bowl of warm water until softened, about 10 minutes.

Drain and set aside.

Strain broth, discard solids, and return to a boil.

Add chopped ginger and scallions, reduce heat and simmer for 5 minutes.

To serve: divide vermicelli into 4 bowls, tops with bean sprouts and shredded chicken, then ladle over broth.

Serve with cilantro, mint, and lime on the side.