Preheat the oven to 300F.
Get out a Dutch oven or roasting pan that is both stovetop and oven-safe and is big enough to hold the brisket.
Pat the brisket dry with paper towels. Season both sides with salt and pepper.
Heat the Dutch oven or roasting pan over medium-high to high heat. Once it’s scorching hot, add the oil, and then the brisket. Sear it on both sides, about 3 to 5 minutes per side, flipping once.
Remove from the heat and add the onion, garlic, and beef stock.
Cover the pan (with its oven-safe lid or with aluminum foil), and roast until the meat is tender enough to cut with a fork, about 6 to 7 hours. Check the brisket periodically (every couple hours or so) and see add a splash more beef stock if necessary.
Cool to room temperature, and then refrigerate overnight (right in the pan it was roasted in). While it’s still cold, remove the fat, which will have collected at the top.
Reheat the meat in the oven or on the stovetop, and serve along with the juices to drizzle on top.