Pour water into a 6-quart Instant Pot. Whisk in garlic, sugar, and vinegar.
2 cups water, 5 cloves garlic, 2 Tbsp. sugar, 2 Tbsp. apple cider vinegar
Rinse corned beef under water to remove excess solution and it to the Instant Pot, fat side-down. Tuck the quartered onion along the sides and sprinkle the pickling spices on top of the beef.
3 ½ lbs. corned beef, 1 medium sweet onion, 1 ½ Tbsp. pickling spice, ¾-1 tsp. salt, ¼ tsp. black pepper
Close the lid and make sure the pressure release valve is in the sealed position. Using the manual function, set the pressure to High and the timer for 80 minutes. Quick release the pressure once the timer goes off.
While the corned beef is cooking, prepare the vegetables by cutting the potatoes into 1-inch chunks, peeling and cutting the carrots into 1-2 inch pieces, and cutting the cabbage into 3-inch wedges.
1 lb. red potatoes, 5 large carrots, 1 small head cabbage
Remove the corned beef from the pot and add in the potatoes, carrots, and cabbage. Place the corned beef back on top of the vegetables, immersing it in some of the liquid. Close the lid again and turn the pressure release valve to the sealed position. Cook for 2 more minutes on high pressure. Quick release the pressure as soon as the timer goes off.
Cut the corned beef against the grain for serving. Season the vegetables with salt and pepper, to taste. Enjoy!