Heat your instant pot to SAUTE. Season the short ribs generously with salt + pepper.
Add the olive oil to the instant pot and brown the short ribs 3 or 4 at a time on all sides until golden brown. About 5 minutes per side. Work in batches so you don't overcrowd the instant pot.
After all the short ribs are browned, remove all but 1 tablespoon of the rendered drippings. To it, add the carrot, celery, onion and garlic to the instant pot. Saute in the drippings. Pour in 1 cup of the stock, and scrape up any brown bits at the bottom.
Add in the rest of the stock, tomato paste, orange juice, orange zest, balsamic vinegar, cinnamon + bay leaf. Stir to combine.
Add the short ribs back to the instant pot.
Close with the lid, and make sure the vent is at SEALING. Hit CANCEL, then cook on HIGH PRESSURE for 45 minutes. Will take about 15-20 minutes to reach pressure, then it will start counting down from 45. After the 45 minutes is up, do a QUICK RELEASE, where you move the valve to VENTING to release all the steam. Once all the steam is gone and the float valve has dropped, you can safely open the instant pot.
Gently remove the short ribs from the instant pot. They will be very tender, and may fall off the bone.
Once all the short ribs are removed, strain the sauce to remove all the vegetables. Discard the cooked vegetables.
If you are making this the day before: Put the short ribs in an airtight container and refrigerate overnight. Put the sauce in a separate air tight container (I use quart mason jars) and refrigerate overnight. The next day, scrape all the hardened fat off the top of the cooking liquid. Return the short ribs and cooking liquid to the instant pot. Press SLOW COOK for 4 hours to reheat.
If you are making this the day of: Return the short ribs to the instant pot. Put the cooking liquid in a fat separator or let it sit in a quart mason jar until the fat settles to the top. Spoon it off and discard. Return the fat skimmed cooking liquid to the instant pot. Press SLOW COOK for at least 30 minutes to rewarm before serving, or up to 4 hours.
Serve warm over mashed cauliflower, with the cooking sauce spooned on top. Garnish with orange zest.
Leftovers will keep in an airtight container refrigerated for at least 5 days, or frozen for at least 6 months.