Place the trivet or steamer rack in the base of your pressure cooker. Sit the corned beef brisket on top, fat-side up.
Scatter the spice packet over the brisket, then nestle the onion quarters and crushed garlic cloves around the meat.
Pour in the beef stock and water so the liquid comes up around the brisket without completely covering it.
Close the lid, set the valve to sealing, select high pressure and cook for 1 hour 30 minutes.
Let the pressure release naturally for 15 minutes, then carefully turn the valve to vent to release any remaining pressure. Remove the lid.
Transfer the brisket to a board, loosely cover with foil and keep warm.
Add the halved small red potatoes and carrot chunks to the cooker. Close the lid and cook on high pressure for 4 minutes, then quick-release.
Add the cabbage wedges, close the lid again and cook on high pressure for 2 minutes, then quick-release once more.
Slice the brisket against the grain and arrange on a serving platter with the potatoes, carrots and cabbage. Spoon over some of the cooking liquid and serve with wholegrain mustard or horseradish, if you like.