Season both sides of the short ribs with the salt and black pepper.
Press the “Sauté” function on the Instant Pot and wait for it to heat up. Add the oil; once hot, add half of the short ribs. Sear on both sides, about 2 to 3 minutes per side. Use tongs to transfer the seared short ribs to a plate. Sear the remaining short ribs the same way.
Add the seared short ribs back into the pot along with the onion and garlic. Cook 2 minutes, stirring as best as you can.
Add the beef broth, vinegar, brown sugar, tomato paste, Worcestershire sauce, bay leaf, thyme, and rosemary and give it a stir.
Press “Cancel” to stop the “Sauté” function.
Cover the Instant Pot, making sure the lid is set to “Sealing”, and cook on “Manual, High Pressure” for 45 minutes.
Once it’s done cooking, let it do a natural pressure release for 15 minutes. After that, carefully release the pressure and then open the pot.
Use tongs to transfer the short ribs to a plate. Tent the plate with foil to keep them warm.