Place potatoes, carrots, cabbage, and onions (in that order) in the bottom of a large Dutch oven. Pour beer or beef broth over top.
Rub meat with brown sugar and the seasonings from the spice packet. Place the beef (fat-side up) on top of the vegetables in the Dutch oven.
Cover and bake for about 3 hours (or longer, if using a larger piece of meat).
Transfer meat to a cutting board and allow to rest for about 5-10 minutes. Thinly slice the brisket against the grain. Serve the vegetables with a slotted spoon.
Notes
You can also prepare this meal in a slow cooker. Cook on LOW for 10-12 hours, or on HIGH for 5-6 hours.
Cut smaller potatoes in half, or chop larger baby potatoes into quarters. You want all of the pieces to be about the same size so that they cook evenly and are tender by the time the meat is done.
Place the brisket in the pot with the fat-side up. The fat will drip down over the meat as it bakes, keeping the beef moist and flavorful.
The meat is done when it's very tender. It has a pink color because nitrite is used in the curing process -- not because the meat is rare. If you're using a larger brisket, you may need to increase the total cooking time.
Slice the beef very thinly against the grain, which means that you want to cut perpendicular to the natural parallel lines of muscle fiber that you see running down the meat.
If you're cooking for just one or two, don't shy away from this dish. You'll want those leftovers! Use the extra corned beef in this Dump-and-Bake Reuben Casserole, or tuck the meat inside a Reuben sandwich (my favorite!).