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1
Cover chicken with 12 cups water in a 6- to 8-quart pot.
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2
Cut off one third of ham (trim if necessary) and cut into 1/3-inch dice, then chill.
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3
Add remaining two thirds ham to pot.
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4
Trim scallions, then cut 1 scallion into thin slices and reserve.
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5
Cut remaining scallions crosswise into thirds and add to pot along with ginger, garlic, and cilantro stems.
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6
Cook chicken, uncovered, at a bare simmer 3 hours (do not let boil), skimming foam as necessary.
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7
(Do not stir, or stock will be cloudy.)
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8
Transfer chicken with tongs to a cutting board, reserving stock.
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9
When chicken is cool enough to handle, remove breast meat and shred into 3-inch-long strips, discarding skin.
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10
Chill strips and reserve remaining chicken for another use.
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11
Pour stock through a fine-mesh sieve into a large bowl, discarding solids and skimming off fat (see cooks' note, below).
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12
Bring 6 cups stock with salt to a simmer.
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13
While stock is coming to a simmer, bring about 2 quarts water to a boil in a 4-quart saucepan.
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14
Soak noodles in a bowl of very hot water until softened, about 10 minutes, then drain in a colander.
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15
Add noodles to boiling water and cook 1 minute, then add lettuce and cook 30 seconds.
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16
Drain in a colander.
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17
Using tongs, divide noodles and lettuce among 6 deep bowls, forming small nests, then top with shredded chicken and diced ham.
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18
Ladle 1 cup hot broth into each bowl and sprinkle with sliced scallion.
