Add half the oil to the pot and wait for the display to read Hot and for the oil to heat up.
Sauté the meatballs in Instant Pot in a bit of oil for a few minutes on each side. No need to cook through. This step is only to add a bit of crust and develop flavour.
Remove the meatballs from Instant Pot and set aside.
Add the rest of the oil to Instant Pot. Add the onions, mushrooms, and garlic to Instant Pot and sauté for another few minutes stirring occasionally.
Deglaze the pot with Vermouth and scrape the bottom of Instant Pot really well to make sure that there are no little bits stuck to it. If there are bits stuck to the bottom of the pot then Instant Pot may show a "burn" message and not come to pressure.
Add the sautéed meatballs on top of the mushrooms, add the broth.
Turn Sauté off. Close the lid. Turn the valve to sealing.
Press Manual or Pressure Cooker button (depending on your model) and using the arrow buttons set the cook time to 5 minutes. Use default High Pressure.Depending on your Instant Pot model, it'll take around 5-7 minutes to come to pressure. Once the Instant Pot beeps that the 5 minutes of cooking are done, do a Natural Pressure Release for about 15 minutes. It means that do nothing for 15 minutes and the pressure will naturally release from the pot. Finish with a Quick Release, this should be very quick. Open the lid.
Add a few tablespoons of hot liquid from Instant Pot into a bowl with cornstarch and mix really well until it's fully dissolved. This is called cosntrarch slurry and it helps thicken the sauce.
Add the cornstarch slurry into the pot.
Add the cream into the pot. Mix.
If Instant Pot automatically turns Keep Warm function on, then press Cancel.
Press Sauté button again and let everything cook for about 1-2 minutes until the mushroom sauce thickens.