Rinse brisket with cold water and thoroughly pat dry.
Place onion, garlic and pickling spice into a 6-qt Instant Pot®. Gently place brisket on top of the onion mixture. Top with beer and beef broth.
Select manual setting; adjust pressure to high, and set time for 85 minutes. When finished cooking, quick-release pressure according to manufacturer’s directions.
Remove brisket from the Instant Pot®; keep warm. Remove and discard onion mixture, reserving 1 1/2 cups cooking liquid in the Instant Pot®.
Stir in potatoes and carrots; top with cabbage. Season with salt and pepper, to taste.
Select manual setting; adjust pressure to high, and set time for 4 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, garnished with mustard and parsley, if desired.