Remove brisket from package and quickly rinse off most of the gel. Do not “scrub” it clean, just get off the majority. If desired, trim some, not all of the fat from the top of the brisket, leaving an even layer.
Place 1 1/2 cup water in the Instant Pot, along with the onion pieces. Position the rack on top and add the brisket, fat side up. Sprinkle the content of the seasoning packet over the top of the brisket, allowing some to fall into the liquid, below.
Seal and set to High Pressure for one hour and 25 minutes (85 minutes.) When finished, use a Quick Release. Remove the brisket and rack. With tongs, place brisket back in the liquid, fat side down, then remove, and set aside, tenting with foil to keep warm. Remove, and reserve one cup of the liquid.
Add the potatoes to the bottom of the Instant Pot, sprinkle with salt, add carrots on top of the potatoes, larger pieces first. To cook the cabbage to a very soft state, add it on top of the carrots. If cooking the cabbage to a firmer state, place a round of foil with the edges turned up about an inch, just slightly smaller than the Instant Pot liner, on top of the carrots and then add the cabbage on top of it. Drizzle the reserved liquid over the cabbage.
Seal the Instant Pot and set to High Pressure for five minutes. When finished quick release.
Slice the brisket, serve arranged on a platter with the vegetables. Strain the liquid if desired and pour some over the brisket and pass the rest on the side.
Note: If a corned beef that is nearly falling apart in chunks is desired, set the timer for 90 minutes.