Season the short ribs liberally with salt and pepper. (If you’d like, you can do this step the night before and store the seasoned ribs in the fridge.)
Take out your pressure cooker or Instant Pot, and melt the ghee or fat over medium high heat. Sear the ribs in batches until well browned and transfer them to a platter.
While the ribs are browning, chop up the veggies and toss the onions, carrots and celery into the empty pot.
Lower the heat to medium, season with salt and pepper, and sauté the vegetables until softened.
Toss the dried porcini mushrooms into the pot along with the garlic. Stir the pot for one minute.
Add in the marinara sauce, broth, and 1 tablespoon of the balsamic vinegar.
Add the ribs back into the pot, mixing well.
Increase the heat to high and bring the stew to a boil. Cover the pressure cooker with the lid and let the contents come to high pressure.
Once the pot reaches high pressure, decrease the heat to low and maintain on high pressure for 30 minutes. Then, take the pot off the heat and let the pressure come down naturally (10-15 minutes).
When the pressure is released, add the remaining tablespoon of vinegar, check for seasoning, and top with minced parsley. You can eat the stew right away, but I think it tastes much better after the flavors have had a chance to meld overnight in the fridge.
Notes
To reheat the stew, dump it in a pot, and bring to a boil. Simmer the stew for at least 20 minutes and top with minced parsley.