Place the corned beef (fat side up), the seasoning packet, and onions in a large Dutch oven. Add 5-6 cups of water. Cover and bring to a boil over medium-high heat, then reduce the heat to a simmer and cook for 2 ½ hours.
Add quartered potatoes and carrots. Simmer, covered, for 15 minutes.
Add cabbage wedges and simmer, covered, for 10-15 minutes.
Remove the corned beef onto a platter, cover with foil, and let rest for 15 minutes. Leave the vegetables in the broth while the meat rests. Note: If you don't have room for the desired amount of vegetables in your pot, you can remove the corned beef from the Dutch oven after 2 ½ hours and simmer the vegetables separately while the meat rests.
Video
Notes
Slow Cooker Method
Step 1: Place the corned beef (fat side up), the seasoning packet, and onions in the slow cooker. Add 3-4 cups of water. Cook on low for 3 hours.Step 2: Add carrots and quartered potatoes. Cook for an additional 5 hours, adding cabbage wedges during the final 1-2 hours. Cook until the cabbage is tender.Step 3: Remove the corned beef, cover with foil, and let it rest for 15 minutes before slicing. Leave vegetables and broth in the slow cooker.Step 4: Cut slices against the grain, or shred if using a point cut of meat. Serve with the potatoes, carrots, and cabbage.Note: I prefer slow-cooking corned beef on LOW heat rather than HIGH heat. If you want a quicker method, cook it over the stovetop or in a pressure cooker.
Instant Pot/Pressure Cooker Method
Step 1: Place the corned beef (fat side up) on the rack or trivet in the Instant Pot. Add 3-4 cups of water, onion, and seasoning packet.Step 2: Cook on HIGH pressure for 90 minutes. Let the pressure release naturally for 15 minutes, then release the valve to let the remaining pressure quick-release.Step 3: Remove the corned beef onto a platter and cover with foil. Leave the liquid and rack in the pressure cooker. Add carrots and potatoes. Place cabbage wedges on top. Pressure cook on HIGH for 4-5 minutes. Quick-release the pressure.Step 4: Slice the meat against the grain and serve with vegetables.