In a large saucepan over medium-high heat, add all spices and toast until they become fragrant. Take care not to burn them! Place them in a coffee filter or piece of cheesecloth and tie it up. Place the sachet into the pressure cooker pot.
In the same sauce pan over medium-high heat, add 1 tablespoon butter (or oil) and add onion and ginger pieces. Brown until there is a nice sear on them. Remove them from the pan and place them in the pressure cooker.
Sear the meat in batches: add the remaining tablespoon of butter to the pan and sear the shank, oxtail and brisket. You'll do this in batches, all in a single layer. Give everything plenty of space so that they SEAR and brown. Crowding the pan will not brown the meat. Sear each side, remove each meat from the pan and add them to the pressure cooker.
Cover the contents of the pressure cooker with water or up to the fill line. Set your pressure cooker to cook for 60 minutes. This means it will cook under pressure for 60 minutes. It will take time to build up pressure (usually 30 minutes) and additional time to release pressure after cooking (usually 30 minutes).
Approximately: 30 minutes to build up pressure + 60 minutes under pressure + 30 minutes to release pressure. Exact timing is really not that important - and also depends on your pressure cooker system. Follow manufacturer instructions.
Once finished cooking and safe to open, open the pressure cooker and using a fine mesh or ladle, remove the top layer of fatty liquid that has accumulated on the surface of the broth and discard (there will be lots of it.) Remove the onion, ginger and spice sachet and discard. Remove the meat to your cutting board. Shred the brisket using two forks. Remove any other meat from bone.
The resulting broth is a concentrate. Dilute the pho concentrate with 4-6 cups of water. Season the Pho broth with fish sauce. Taste and add additional fish sauce if needed. Bring to a simmer on stovetop right before you are ready to serve.