The best way to cook corned beef is slowly. You can bake, boil, or cook it in your slow cooker, but the key to a tender, flavorful meal requires at least a few hours. All of these methods are very easy and you can add vegetables to the meat while it's cooking to create a one-dish dinner.
It is important to note that corned beef is often sold pre-cooked. If that's the case, you'll just need to heat it up. These methods are used if you brined it yourself or purchased a piece of cured but still raw corned beef.
Boiling is the traditional way of preparing corned beef. This approach accomplishes three things simultaneously: It cooks the meat, tenderizes the tough cut of brisket usually used to make corned beef, and draws out the excess salt. While the salt used in brining is responsible for corned beef's delicious flavor and texture, it can be too salty if some of it isn't removed.
To boil corned beef, put the meat in a large pot and cover it with plenty of water. Add a tablespoon or two of pickling spice as well as a few garlic cloves, a quartered onion, a carrot, and a few stalks of celery if you like. Bring everything to a boil, cover, and reduce the heat to maintain a steady simmer. Cook (don't disturb it much) until the corned beef is completely tender when pierced with a fork (about 3 to 4 hours). Remove the corned beef from the pot and slice across the grain to serve.
You can also add potatoes and/or cabbage about 30 minutes before the end of the cooking time. Alternatively, wait until the corned beef is done, then pull it out of the pot and cover it to keep it warm or set it in a 200 F oven. Cook whole small red potatoes and cabbage cut in wedges in the liquid until they are done.
Cooking corned beef in a slow cooker has all the benefits of boiling it but you don't have to stay home while it cooks. Place a quartered onion, a carrot, a couple of garlic cloves, and a few stalks of celery in the slow cooker. You can also add potatoes. Set the corned beef on top of the vegetables, sprinkle it with a tablespoon of pickling spice, and add just enough water to submerge the beef. Cover and cook on high for about 4 1/2 hours or on low for 8 to 9 hours.
Baked corned beef develops a fabulous crispy crust. Before you bake it, however, you really must boil it to remove some of the curing salt. Place the corned beef in a large pot and cover with water. Bring to a boil while you also preheat the oven to 350 F. Discard the water and repeat to draw out more of the salt.
Set the corned beef fat-side-up in a baking pan and cover with foil. Bake for 2 hours. Unwrap, position an oven rack in the top third of the oven, and bake until the top is browned and crispy, about 30 minutes. Some people like to cover the meat with mustard or sprinkle it with brown sugar (or both) to add flavor and help the brown crust develop.