Slice your onion, large slices and put into your pressure cooker on bottom. Place your chuck roast on top (cut in half if you need to to fit properly with lid closed).
In a separate bowl whisk together all other items until smooth and pour on top of your meat.
Close your pressure cooker lid and close steam valve. Set to high pressure for 90 minutes, then allow to naturally release for 10 minutes.
Lift lid and either shred beef in pot or I find it easier to remove and put into a bowl so I can separate the fat and bone and discard those and shred beef well.
If you want sauce inside pot to be thicker turn pot to saute now. In a small bowl add 2 tbsp cornstarch with 4 tbsp of hot liquid and whisk until smooth, then add this into the pot. Stir slowly and allow to thicken for about 2-3 minutes. Then turn pot off.
Add shredded beef back into pot so it soaks up sauce. Serve over rice, in tortillas for soft tacos, or on slider buns topped with macaroni salad!