Slice onion and mushrooms and saute in oil for 5 minutes in a large pot or pressure cooker.
Add the barley and liquid.
Cook for half an hour, or 10 minutes in the pressure cooker after it has reached full steam. Let the cooker cool off before opening to avoid barley starch shooting all over the room. If you don’t have time for this, cook everything together, except for parsley) for 20 minutes and then let depressurize naturally.
Add sliced carrots, sliced celery, garlic, and Tamari sauce. Cook again for 30 minutes, or 10 in the pressure cooker.