In a Dutch oven, place corned beef, seasoning packet, onions, beer (or juice), and water (to cover). On high heat, bring to a boil. Reduce to a simmer and cover.
Simmer for 2-2 ½ hours, or until meat is tender.
Remove beef from the cooking liquid and set aside to rest.
Add carrots and parsnips, if using, to the Dutch oven. Add water to cover the vegetables if needed. Cover Dutch oven and simmer for 10 minutes.
Add potatoes and cabbage to the Dutch oven, cover, and simmer for 15-20 minutes, or until vegetables are tender. Remove vegetables from liquid*.
Rest meat for 15 (or more) minutes.
Optional Wet Rub
While beef is resting, preheat oven to broil. Grease a roasting pan.
Combine brown sugar and mustard to create a wet rub. Add beef to roasting pan and slather with the rub. Broil for 2-5 minutes, until rub is lightly browned. Watch carefully.
Cut into ½ inch slices, cutting across the grain. Serve with cooked vegetables and potatoes.
Notes
*Place on a serving platter or a baking tray with a cooling rack to drain while preparing the rest of the meal.
Rest corned beef for at least 15 minutes so the juices can recirculate back into the meat.
Slice corned beef against the grain to further tenderize the meat.