Bring a large pot of water to a boil over high heat.
Add the chicken, scallions, ginger, and salt and boil for 15 minutes, then turn off the heat, cover the pot, and let stand for 25 minutes.
1 4 to 5 lb whole chicken, 6 whole scallions, 1 thumb-sized piece of ginger, 1 tbsp Kosher salt
Just before the chicken is ready, prepare an ice-water bath in a large bowl.
When the chicken is done, carefully use tongs and a large spoon to remove it from the pot and immediately submerge it in the ice-water bath, which will stop the cooking and give the meat a firmer texture. Discard the cooking liquid, or store (chill or freeze) it for another use.
Let the chicken chill for about 20 minutes, or until the chicken is cool enough to handle easily, remove from the water, and pat dry.
Pull the chicken meat from the bones, discarding the bones and skin. Shred the meat with your fingers; you should have about 4 cups (this step can be done a day ahead).
In a large saucepan, bring the stock to a simmer over medium-high heat. Taste and add more salt (or fish sauce) until you reach the correct balance of seasoning.
3 quarts chicken stock
Prepare the rice noodles according to package directions and distribute them evenly among soup bowls.
1 16 oz. package dried rice vermicelli
Prep the garnishes and arrange the basil, bean sprouts, lime wedges, jalapeños, and fried shallots on a platter and place them on the table.
1 bunch scallions, 1½ cups cilantro, Crispy fried shallots, Thai, or Chinese basil, Mung bean sprouts, 2 Limes, 2 Jalapenos
Top each serving with about ¾ cup of the shredded chicken, then divide the scallions and cilantro evenly among the bowls.
Ladle the hot stock over the top, dividing it evenly, and sprinkle with the fried shallots.
Serve at once, accompanied with the platter of garnishes.