Season the short ribs with 2 teaspoons of salt and a teaspoon of pepper.
Set the Instant Pot to the ‘Sauté’ mode on ‘High’. Once the display reads "HOT, add 2 tablespoons of oil and heat it up.
Sear the ribs, 4 minutes each side, until they're well browned. Remove from the pot and set them aside. (TIP: If the meat doesn't detach from the pot easily, don't force it. Simply wait for 30 seconds or so and try again. Well seared meat detaches without any effort.)
Add the remaining tablespoon of oil to the pot. Then add the onions, carrots, celery and garlic and sauté for 3 to 4 minutes or until the vegetables soften.
Add the wine to deglaze the pot and scrape the bottom to loosen up any meat that stuck to the bottom of the pot. Let the wine simmer for 2 minutes.
Next, add the beef stock, balsamic vinegar, remaining teaspoon of salt and pepper, dried thyme and bay leaf. Add the seared short ribs back to the pot.
Put the lid on, making sure that the pressure valve is in the “sealing” position.
Select the “Manual" or "Pressure Cook” mode. Adjust the time, with the “- / +” to 45 minutes.
When the time is up, let it release the pressure naturally for 20 minutes, then turn the pressure valve to “venting” position to do the quick pressure release.
Open the lid and carefully take out the beef short ribs. Cover tightly with foil to keep warm.
Using mitts, carefully strain the braising liquid and discard the vegetables.
Turn the Instant Pot back on ‘Sauté’ mode on ‘High’. Add the strained braising liquid through a fat separator back to the pot and discard excess fat. Bring the liquid to a simmer.
In a small bowl, mix cornstarch with water to make a slurry. And add the slurry to the braising liquid.
Let it cook for a minute or until the sauce thickens.
Add back the short ribs and toss gently to coat with the sauce. Serve immediately with fresh parsley, if desired.