Remove the bone and fat from the arm roast and cut it into large chunks. Place the meat into a pressure cooker and sprinkle the top of the meat with salt.
Slice the onion and scatter on top of and around the meat.
Pour in the remaining ingredients except the cornstarch and close the lid. Seal the steam valve and set the pressure cooker to high pressure for 45 minutes.
Allow the steam to naturally release for 15 minutes, and then release the rest of the steam.
Carefully lift the lid and transfer the meat and onions to a large bowl. Set the pressure cooker to saute.
In a small bowl mix together the cornstarch and water to make a slurry.
Add the slurry to the pot and continuously stir for about 3 minutes until the sauce thickens.
Shred the meat with two forks. Pour the sauce on top of the meat and toss to coat well. Serve with your favorite sides and enjoy!