Place trivet at the bottom of the instant pot, and place brisket (2.5 pounds) on top. If using a brined brisket, make sure to rinse before adding it. Pour water over the brisket, followed by the bay leaves (3 torn), peppercorns (1 tablespoon), coriander seeds (1 tablespoon), mustard seeds (1 tablespoon,) and garlic cloves (4).
Place lid on top and make sure the pressure release valve is set to "sealing". Set IP to high pressure for 90 minutes. Once done, turn the pressure release valve to "venting". Once the pin drops, remove the lid.
Check the corned beef, it should be fall apart fork tender - if it's not, don't be afraid to cook it for another 10 minutes at a time until it is.
Use tongs to remove the corned beef to a large cutting board. Use a slotted spoon to remove and discard the aromatics. Spoon some of the cooking liquid over the corned beef, then loosely tent it with foil.
Add the carrots (2), cabbage (1), and onion (1) to the IP. Seal and cook on high pressure for 3 minutes. This may take longer if you have a very large head of cabbage, so check and continue to cook until you've reached your desired doneness. When done, turn the pressure release valve to "venting", and remove the lid only after the pin has dropped.
Slice the corned beef against the grain before serving. Sprinkle with chopped parsley (1/4 cup), and serve with a side of the cooking liquid. Enjoy!