Press the SAUTE button on the Instant Pot. As soon as it's hot, add the oil.
Pat the ribs dry with a paper towel, then season all sides with salt and pepper.
Add ribs to hot oil in Instant Pot (you might have to do it in batches - you want them in a single layer) and sear on all sides (about 1-2 minutes per side - you aren't cooking the ribs, just browning them). Remove ribs from pot and place on a plate off to the side.
Add beef broth to pan and using a wooden spatula or spoon, deglaze the bottom of the pot (scraping up any browned bits that cooked to the bottom of the pan).
Add ribs back into pot. In a mixing bowl, combine French onion soup and chili sauce. Pour sauce over ribs.
Place the lid on top of the Instant Pot and move the valve to SEALING. Press the PRESSURE COOK (MANUAL) button and set timer for 45 minutes.
When the timer is done, let the pressure do a NATURAL RELEASE (should take about 15-20 minutes).
Remove lid and carefully remove short ribs.
If desired, you can thicken up the sauce and drizzle it on top of the ribs. To thicken up the sauce, press the SAUTE button after the ribs are removed.
In a small bowl, mix together the cornstarch and cold water. Add the cornstarch slurry to the pot and whisk it into the liquid. Let it cook until the sauce has thickened to the desired consistency you want. Serve sauce over ribs, top with rosemary and enjoy.