Step 1

Preheat the oven to 350°. Place the corned beef, fat side up, on a rack in a roasting pan. Spread the mustard over top, and sprinkle the meat with the contents of spice packet. Editor's Tip: If you don't have a roasting pan with a rack, use an oven-safe rack set inside a rimmed baking sheet. Alternatively, you can set the corned beef on top of a bed of chopped onions so it's elevated from the pan.

Step 2

Add 1/2 inch of water to the pan. Tightly cover the pan with foil. Bake until the meat is tender, two to three hours. If necessary, add additional water during cooking. Editor's Tip: If you have a meat thermometer, cook the corned beef until it reaches an internal temperature between 180° and 195°.

Step 3

Let the corned beef stand for 10 minutes. Then, cut it diagonally against the grain into thin slices.