Place the charred onions and smashed ginger into the cooker.
Place the 2 halves of chicken atop of the vegetables.
Add seasoning bag, rock sugar and salt. Pour water in the cooker.
Pour water in the cooker. Place the lid on. Pressure cook on high for 17 minutes and natural release for 25 minutes (exercise caution when releasing pressure from the cooker). Follow the manufacture instructions for how to do a quick release. Remove lid. Press the cancel button.
Remove and discard the onions, ginger and seasoning bag.
Remove chicken from the cooker and place in a bowl. Debone chicken and remove skin (optional). Shred or cut the chicken meat and set aside until ready to eat. Discard the bones.
In a large bowl or pot: place in a mesh strainer lined with a coffee filter or cheesecloth. Slowly pour the stock through the filter. Strain to remove all the vegetables and meat solids. This will help to significantly reduce the fat which leaves a clear pho broth. Clean the inner pot and place the pot back in the IP. Transfer the broth back in the cooker.
Press the saute button and set to high ~20 minutes.
Pour fish sauce in the broth then stir and bring it to a boil.