Prepare the pork loin by rubbing in the Italian Seasoning and crushed garlic. Heat the oil in the pressure cooker and put in a few of the onion pieces to flavor the oil leaving it just long enough to soften slightly, then remove it and set aside. Brown the roast on all sides and both ends. Remove to extra plate so that you can add 2/3rds of the chicken broth scraping the bottom of the pan to get up all the good bits; add the trivet and return the roast, fat side up. Add the onion. Pressure cook for 1 hour; remove from heat and let it the pressure come down on its own.
Remove the roast and onions to a serving dish and tent with foil to rest for 15 minutes. In the meantime, mix the cold water and corn starch in a mixing cup or small bowl (I like to add a little pepper here). Remove the trivet from pot, add the rest of the chicken broth and bring to a boil. Add the water/corn starch mixture to the pot and remove from heat. This makes a wonderful gravy for the roast.