Using the Sauté function, sear the roast in the oil for 3 minutes on each side until browned all over.
Turn off the pot and add 1 cup of water, plus the ranch mix, au jus mix, and pepperoncini.
Seal the lid, select the Pressure Cook function, and cook on high pressure for 40 minutes.
When the timer goes off, allow the pressure to release naturally for 10-15 minutes, then use Quick Release to vent the remaining pressure.
Remove the lid, add the carrot and potato pieces, then reseal and pressure-cook on high pressure for another 20 minutes.
Make a slurry by combining 1/2 cup of water and the cornstarch.
When the timer goes off (again), allow the pressure to release naturally for 15 minutes, then use Quick Release to vent the remaining pressure.
Remove the roast and veggies to a plate and skim any fat out of the pot.
Use the Sauté function to make gravy by slowly whisking the cornstarch slurry into the liquid at the bottom of the pot.
Serve the pot roast, carrots, and potatoes warm with the gravy.