Begin by heating a large nonstick skillet over medium-high heat and adding the olive oil. Season both sides of the pork chops with garlic powder, salt, and pepper. Sear the chops for approximately 2 to 3 minutes per side, until they achieve a golden-brown crust.

While the chops are searing, pour 1 cup of chicken broth into the Instant Pot, followed by the diced onion and sliced mushrooms. Place the seared pork chops on top of the vegetables, and then spoon the cream of mushroom soup over the chops. Close and lock the lid, then select the high-pressure setting and cook for 15 minutes, allowing 10 to 15 minutes for the pressure to build.

Once the cooking cycle is complete, release the pressure using the natural-release method for at least 10 minutes. Afterward, use the quick-release method to release any remaining pressure, taking approximately 5 minutes. Carefully unlock the lid and remove it.

Remove the pork chops from the pot and set them aside, keeping them warm. To thicken the sauce, whisk together the cornstarch and remaining 1/4 cup of chicken broth until smooth. Stir this mixture into the liquid in the Instant Pot and set it to sauté. Bring the sauce to a simmer, whisking occasionally until it reaches the desired thickness. Serve the pork chops with a generous portion of the mushroom and onion sauce.