Add the beef broth, onion, and garlic to the insert of a 6- to 8-quart Instant Pot. Place the brisket on top of the mixture, fat side up. Pour the beer over the top and sprinkle the corned beef with the pickling spices. Close and lock the lid.
Select MANUAL or PRESSURE COOK setting, toggle to HIGH, and set the time for 1 hour and 25 minutes.
When the cooking time is complete, allow the Instant Pot to do a natural release for 10 minutes and then manually release any remaining pressure.
Meanwhile, slice the core end of the cabbage off and place the cabbage cut side down so you have a stable base. Slice the cabbage in half and then cut each half in quarters. You should end up with 8 chunks of cabbage. Slice the potatoes in half. Peel and cut the carrots into large 3-inch chunks.
Transfer the corned beef to a cutting board and tent with a piece of foil to keep it warm. Use a slotted spoon to remove and discard the onions, leaving the cooking liquid behind in the Instant Pot. Add the potatoes and carrots to the Instant Pot and place the cabbage wedges on top. Be sure not to fill the Instant Pot more than 2/3 full. Close and lock the lid.
Select MANUAL or PRESSURE COOK setting, toggle to HIGH, and set the time for 5 minutes. When finished cooking, quick release the pressure.
Slice the fat layer off the top of the corned beef (flat briskets will have far more fat than a round) and then slice the corned beef against the grain and transfer to a serving platter. Drizzle the corned beef with a little of the cooking liquid.
Use a slotted spoon to transfer the potatoes, carrots and cabbage to the serving platter with the corned beef or to a separate bowl. Toss the veggies with butter, season to taste with salt and pepper, and garnish with parsley, if desired.
Serve with grainy mustard on the side.