Preheat your oven to 300°F (150°C). Pat the corned beef dry with paper towels, then rub with vegetable oil, salt, and pepper.
Arrange the carrots, potatoes, onion, and garlic in a roasting pan. Sprinkle the included spice packet evenly over the brisket, then place the meat fat side up on top of the veggies.
Pour beef broth (or water) into the pan to keep everything moist. Cover tightly with aluminum foil to seal in steam.
Roast in the preheated oven for 3 hours, allowing flavors to meld and the meat to become tender.
Carefully remove the foil, then continue roasting for an additional 1 to 1½ hours until the corned beef is fork-tender and the vegetables are golden.
Transfer the brisket to a cutting board; let it rest for 10 minutes so the juices redistribute.
Slice thinly against the grain and serve alongside the roasted vegetables.