Rinse brisket with cold water and thoroughly pat dry.
Place trivet into a 6 or 8 quart electric pressure cooker. Place sliced onions into pressure cooker, and place brisket on top of the onion mixture. Top with garlic and pickling spices. Pour in 2 cups of beef broth.
Select manual/pressure cook setting on high pressure. Set cook time for 70-90 minutes (see note below).
Once cook time has elapsed, let pressure release naturally for 5 minutes. Then do a quick release of any remaining pressure - there will be some steam left when you quick release.
Remove brisket from the Instant Pot and place on serving platter. Cover with foil to keep warm.
Reserve 2 cups cooking liquid in the Instant Pot, discard the remaining liquid.
Add baby potatoes and carrots and top with cabbage. Season with salt and pepper, to taste.
Cook on high pressure for 3 minutes. When finished cooking, quick-release pressure according to manufacturer’s directions.
Thinly slice corned beef against the grain and serve with potatoes, carrots and cabbage. Garnish with fresh parsley, if desired and serve with stone ground mustard.