Place the parboiled marrow followed by the tendons, brisket, charred onions, and smashed ginger into the pressure cooker. Stack them according to the above order
Add 12 cups of boiling water into the pressure cooker (which will help establish optimal pressure faster). Keep the water under the 2/3 max line (Do not exceed the max line).
Add the seasoning bag on top of the meat (not in the water), followed by the rock sugar and salt.
Place the lid in position and pressure cook on high for about an hour and natural release for half an hour. (Always be cautious when releasing pressure from the pressure cooker)
Once safe, open the cooker and remove the tendons and brisket. Transfer them to a bowl of cold water and let them rest for 15 to 20 minutes or until cool to touch. Remove or discard the bones, ginger, and onion.
Add 8 cups of hot boiling water to the pressure cooker and remember not to exceed the 2/3 max line.
Place the lid in position and pressure cook on high for about 3 minutes, followed by a quick release.
Add the fish sauce to the broth and stir well (season with salt and sugar if necessary). Discard the bag of seasoning.
Optional, although recommended – place a mesh strainer lined with a cheesecloth or coffee filter over a large pot or bowl and slowly pour the stock via the filter. This will reduce the fat and leave a clear pho soup.
Transfer the broth back to the pressure cooker (ensure you clean the instant pot cooker before reusing it).
On the instant pot, press the sauté button and set it to high for 20 minutes.
Add the meatballs to the broth.