Thinly slice 1 large onion. Trim off the surface fat from a (3-pound) beef brisket if the fat is more than 1/4-inch thick. Cut the brisket in half if needed to fit in a 6-quart or larger electric pressure cooker or Instant Pot. Season the brisket all over with 2 teaspoons of the kosher salt and 1/2 teaspoon black pepper.

Heat 1 tablespoon vegetable oil in the pressure cooker with the highest Sauté function until shimmering. Add the brisket (sear in 2 batches if needed) and sear undisturbed until browned on the bottom, 5 to 6 minutes. Flip the brisket and brown on the other side, 5 to 6 minutes more. Transfer to a large plate.

Add the onion and remaining 1/2 teaspoon kosher salt to the pressure cooker and sauté until the onions are tender, scraping brown bits from the bottom of the pot, 4 to 6 minutes.

Stir in 1/3 cup beef broth, 1/3 cup red wine vinegar, 1/3 cup ketchup, 1/4 cup packed light brown sugar, 1 teaspoon dried thyme, 1 teaspoon dried oregano, 1 teaspoon paprika, and 3/4 teaspoon garlic powder. Return the brisket to the pot, overlapping the pieces if needed.

Lock the lid on and check that the valve is set to seal. Set to cook for 70 minutes under HIGH pressure. It will take 5 to 7 minutes to come up pressure.

When the cook time is up, let the pressure naturally release, 20 to 25 minutes. Carefully open the pressure cooker and transfer the brisket to a cutting board.

Slice the brisket across the grain into 1/4-inch-thick slices. Serve the brisket with the onions and gravy. Garnish with fresh parsley, if desired.