Turn the instant pot on saute mode and add the oil. Allow the oil to sizzle before adding the meat.
You may need to slice the brisket in half to fit. Make sure you cut it in half against the grain if so.
Place the brisket into the instant pot and sear on all sides for about 2-3 minutes per side, and rotate once it easily releases.
Place the brisket fat side down in the instant pot and then top with the prepared vegetables. (2 large onions, 5 carrots, and 4 potatoes, all copped and sliced)
Add the 1 cup of ketchup, 1/2 cup brown sugar, 2 cups seltzer, and onion soup mix to a bowl and stir until combined. Then pour into the instant pot.
Place the lid onto the instant pot securely and turn it on manual high pressure for 70 minutes. Once the time is up, let it naturally release for at least 20 minutes, and then carefully release the pressure after the time has elapsed.
Carefully remove the vegetables with a slotted spoon and place them onto a platter. Remove the brisket and allow it to rest for 10 minutes before slicing it on a cutting board. Remove the fat layer and discard. Slice the brisket against the grain and then place the slices onto the platter with the cooked vegetables.
Carefully pour a ladle full of the cooked broth onto the brisket and garnish with freshly chopped parsley if desired. Serve and enjoy.