Scorch the ginger and the onion pieces, either over a glass flame, using tongs to hold the pieces in the flame until they scorch, or together in a dry heavy skillet over high heat. Turn the pieces until they are blackened on all sides, set aside
Bring a large pot of water to a vigorous boil, drop in the rice noodles, and cook just until softened but not mushy, 30 seconds to 1 minutes. Drain, rinse with cold water, and set aside
Peel and thinly sliced the yellow onion and soak them in ice cold water so that they are crispy. Set aside
Using Instant Pot:
Place the whole chicken on the trivet. Pour in 2 cups of water. Close the lid and turn steam release handle to seal. Press pressure cooker, high pressure and set the timer to 20 minutes. Once the timer is up, carefully release the pressure immediately and wait until the pressure valve collapses entirely and then unlock the lid. Very carefully, lift the chicken out from instant pot to a large plate. Let it cool down a little bit
When cool enough to handle, cut the meat off from the breast, thighs, and remove drumsticks. Don't cut into smaller pieces just yet. Return the carcass and the wings back to the instant pot. There will be drippings from cooking the chicken earlier too. Keep those! Add another 4 cups of water. Add the spices ingredients, charred onion and ginger. Close the lid and press on pressure cooker, high pressure and set timer to 15 minutes. When the time is up, wait 10 minutes and then release pressure. Proceed to "before serving" step
Cooking on Stove-top:
If using a whole chicken, rinse the chicken, including the heart, neck, and giblets, thoroughly with cold water (reserve the liver for another use). Place in a large pot and add the water and spices ingredients . If using leg quarters/thighs, rinse chicken and place in a large pot and add the water. Bring to a boil over high heat
Once the water comes to a boil, skim off the foam, lower the heat, and let simmer, partially covered, until the chicken is cooked, about 30 minutes, skimming off the foam occasionally. If the chicken is not completely covered with water, turn it several times during cooking. Carefully remove the chicken from the broth and rinse with cold water to stop the cooking. Remove the meat from the bones in big chunks but don't cut into pieces yet. Add the remaining bones and wings back into the soup base. Continue to cook the soup base on gentle simmer for the next 2 hours. Proceed to "before serving" step
Before serving:
Line a colander with a double layer of cheesecloth or use a stock strainer and strain the broth into a bowl. Let cool, then refrigerate, covered, for at least 3 hours. When the broth has chilled completely, skim off the layer of fat on the surface with a large spoon and reserve it for use in the soup or for another purpose
When ready to serve:
About 30 minutes before you wish to serve the soup, remove the broth and shredded chicken from the refrigerator. Place the soup in a pot, add the fish sauce, sugar and pinch of salt and bring to a boil, then lower the heat and let simmer until ready to serve. Taste for seasoning and add fish sauce or salt to taste if necessary
To serve, divide the noodles among six large soup bowls. Place about 1/4 cup bean sprouts in each, then top with the chicken. Ladle the hot broth over. Sprinkle in some coriander, basil leaves, onion slices. Squeeze some lime juice over. Squirt in some sriracha chili sauce and hoisin sauce if you wish. Serve at once, with small plates of the remaining bean sprouts, herbs, peppers