Add the chicken carcass/bones to the Instant Pot liner.
In a dry frying pan over medium heat, lightly toast the cinnamon sticks, star anise, coriander seed, cloves, fennel seed, and cardamom pods, a few minutes.
Once aromatic, add the spices to a cheesecloth and tie it up with kitchen twine.
Drop the cheesecloth bundle into the pot.
Using the same dry frying pan, turn up the heat to high and add the halved onion and ginger and char until blackened.
Transfer the charred onion and ginger to the pot.
Add in sea salt, sugar, fish sauce and cover with water.
Place the lid on, set the vent to SEAL and press the SOUP function -- 45 minutes at HIGH PRESSURE.
Let the pressure naturally release.
Remove the lid and take out the spice bundle and bones.
Strain the broth through a sieve into a large pot and skim off any fat.
Give the soup a taste and adjust the seasoning to your liking.