Rinse the corned beef briefly if desired, then pat it dry. Leave the fat cap in place during pressure cooking for flavor, since excess fat can be trimmed after the brisket has cooked.

Pour the broth or water into the Instant Pot, then add the onion, garlic, bay leaf, peppercorns, optional mustard seeds, and the seasoning packet. Set the brisket into the liquid with the fat side facing up.

Lock the lid, set the valve to sealing, and cook on High Pressure for about 90 minutes for a 3 to 4-pound brisket. If using a smaller piece, begin checking the timing range of 70 to 80 minutes.

When the cooking cycle ends, allow a natural pressure release for 10 to 15 minutes before carefully releasing any remaining pressure. Remove the brisket, cover it loosely with foil, and let it rest while the vegetables cook.

Add the halved potatoes and carrot pieces to the seasoned cooking liquid. Arrange the cabbage wedges on top, lock the lid, and cook on High Pressure for 3 to 5 minutes.

Perform a quick pressure release as soon as the vegetable cooking time ends. Open the lid away from your face and transfer the vegetables carefully so the cabbage wedges remain intact.

Trim away excess fat from the rested brisket, then slice it thinly across the grain. Serve the slices with potatoes, carrots, cabbage, and spoonfuls of the cooking liquid, finishing with chopped parsley if desired.