Preheat your oven to 300°F. Rinse the corned beef under cold running water, then place it in a large Dutch oven. Sprinkle the corned beef seasoning over the corned beef, and pour in the water until the meat is fully submerged.
3 pound corned beef (red or gray), 16 cups cold water
Bring the water to a boil over medium-high heat on the stove, skimming off any foam that rises to the surface. Once boiling, cover the Dutch oven with the lid and transfer it to the preheated oven. Let the corned beef cook for about 3 ½ hours or until it's very tender.
While the corned beef is cooking, prepare your vegetables by cutting the cabbage, carrots, potatoes, and onion into large, equal-sized chunks.
½ head green cabbage, 5 whole carrots, 3 large Yukon Gold potatoes, 1 large yellow onion
When the corned beef is tender, carefully remove it from the pot and place it on a cutting board or rimmed baking sheet. Tent it with foil to keep it warm. Add the vegetables to the cooking liquid in the Dutch oven and bring it to a boil over medium-high heat. Reduce the heat to low and let the vegetables simmer, uncovered, for about 20 minutes or until they're fork-tender.
Slice the corned beef across the grain and transfer it to a serving platter. Use a slotted spoon to transfer the cooked vegetables to the same platter. Ladle some of the cooking liquid over the meat and vegetables for extra flavor. Serve with mustard, horseradish, and Irish soda bread.