Thinly slice ginger (no need to peel), peel and smash garlic, cut green onion into 3" pieces.
Place all ingredients into a large pot. Fill pot with water to cover ingredients (~14 cups).
COOK IN PRESSURE COOKER:
Set pressure cooker to cook under high pressure for 30 minutes. (It will take time to bring up to pressure. Once it is at pressure, cook 30 minutes. Let pressure release naturally or release the steam following manufacturer's instructions)
COOK ON STOVE:
Bring pot to boil, then turn heat to low. Cover pot, leaving a little opening (I use a wooden spoon or spatula to prop up lid). Simmer for 2 hours.
COOK IN SLOW COOKER:
Set slow cooker to low for 6-8 hours or high 4-6 hours
When the soup is done, skim the top and discard. Use a slotted spoon to remove the chicken parts, ginger, garlic and green onion (discard). Season with salt or fish sauce to taste (start with 1 tablespoon and add more if needed)