Salt & pepper the short ribs, then add oil to the InstantPot and set to "Saute." When the oil is heated, brown the short ribs a few minutes on each side, in batches if necessary (careful not to burn, as the InstantPot gets hot fast on Saute depending on your model)
Turn off the heat, remove browned short ribs to a plate and set aside. Carefully scoop or pour out excess oil / fat from the pot.
Add soy sauce, broth, sugar, garlic, and ginger to the pot. Stir gently to mix, scraping up any browned bits off the bottom.
Place the short ribs back into the pot so that they sit in the sauce mixture. Add in mushrooms, then layer the potato halves on top.
Cook on the "Meat / Stew" pressure setting for 45 minutes. Do natural release for at least 20 minutes and carefully remove the lid after venting fully.
Stir in frozen peas until heated through. Serve with the potatoes (which can be cut just a little smaller and kept in the pot, or mashed up separately on the side), rice, or a hunk of bread. Garnish with optional cilantro or scallions and a drizzle of sesame oil on top if desired.
Optional but recommended: de-fat the dish if you're able to make it in advance, or use a fat separator (see reader comments!). This will take extra time and effort but we always try to do it! After opening up the Instant Pot, use tongs or a large slotted spoon to remove all the potatoes and short ribs from the sauce and place them in a separate bowl. Let the sauce cool and then chill in the fridge, and skim off and discard the white layer of fat on top. Reheat everything together when you're ready to eat.