Press the sauté button to use the sauté mode on the instant pot. Once it is hot, add in the oil. Add in the pork and cook until all of the sides are browned. You are just browning the pork make sure not to fully cook it. Pull the pork pieces out of the pressure cooker and put into a bowl or on a plate.
Add the onions into the instant pot and sauté for 3-5 minutes. Add the minced garlic cloves, brown sugar, whole black peppercorns, pepper, and dried bay leaves. Sauté just for about a minute. Scrape any browned bits that may have cooked to the bottom of the pot. Add the pork back in.
Pour in vinegar, soy sauce, and water. Scrape any browned bits that may have cooked to the bottom of the pot. Add the pork back in, giving it a quick stir and making sure that the pork is all partially submerged in the liquid.
Put the lid on the pressure cooker, turn the venting knob to the sealing position. Pressure cook for 15 minutes at high pressure. Allow it to natural release for 10 minutes. After that release pressure with a quick release by turning the venting knob to the venting position. Carefully open lid.
If you would like to thicken the sauce, pull the pork meat out with a slotted spoon into a serving bowl. Turn on the sauté function again and bring the adobo sauce up to a simmer. Mix 2 1/2 tablespoons of cornstarch with 3 tablespoons of water in a small bowl. Mix that into the hearty stew liquid. Taste the sauce, season with more soy sauce or salt if needed. Add the pork chunks back in.
Pull out the bay leaves and peppercorns if you like. Serve with white rice.