Prepare the brisket: Cut the brisket in half against the grain to fit into the Instant Pot. Generously rub kosher salt all over the meat.
Sear the brisket: Set the Instant Pot to “Sauté” mode on High or More setting. Once hot, sear each piece for about 2 minutes per side until browned. Remove and set aside.
Make the sauce: In a mixing bowl, whisk together the water, ketchup, brown sugar, vinegar, and minced garlic until well combined.
Layer the brisket and onions: Place the trivet inside the Instant Pot. Lay one piece of brisket fat-side up, cover with half of the onions and sauce. Repeat the layering with the second piece of brisket and the remaining onions and sauce.
Pressure cook: Secure the lid, set the valve to “Sealing,” and pressure cook on High. Choose 75 minutes for extra tender, shreddable meat or 65 minutes for firmer slices. Let the pressure release naturally for 20 minutes, then perform a quick release.
Rest the brisket: Carefully transfer the brisket to a carving board, fat-side up, and let it rest for at least 15 minutes before slicing. If desired, trim any excess fat.
Thicken the sauce: While the brisket rests, mix the cornstarch with 3 tablespoons of cold water to form a slurry.
Simmer the sauce: Set the Instant Pot back to “Sauté” on High. Stir in the slurry and simmer for about 30 seconds until the sauce thickens. Keep warm.
Slice the brisket: Once cooled, slice the brisket against the grain into strips or chunks for the best texture.
Marinate and serve: Return the sliced brisket to the Instant Pot and let it soak in the sauce for 5–10 minutes on the “Keep Warm” setting before serving.