Before baking, bring the corned beef to a boil in a large pot covered with water. Add the spice packet and let it simmer for 90 minutes. This step pre-tenderizes the meat and removes excess salt before the oven does its magic. Don't skip this — it's what ensures the final result is tender all the way through, not just on the surface.

While the brisket simmers, whisk together Dijon mustard, whole grain mustard, brown sugar, apple cider vinegar, garlic powder, and black pepper in a small bowl until smooth and combined. This glaze is tangy, sweet, and slightly sharp — it's going to caramelize beautifully on the surface of the brisket.

Preheat oven to 325°F (165°C). Remove the partially cooked brisket from the pot and place it fat-side up in a roasting pan or baking dish. Scatter chopped onion and garlic around it. Pour beef broth into the bottom of the pan. This small amount of liquid keeps the bottom of the roast moist during the oven time without steaming the whole surface.

Spoon or brush the mustard-brown sugar glaze generously all over the top and sides of the brisket. Don't be shy — coat it thoroughly. This glaze is going to caramelize into a gorgeous, flavorful crust as it bakes. This is the moment where baked corned beef separates itself from every other method.

Cover the roasting pan tightly with aluminum foil and place in the preheated oven. Bake covered at 325°F for 1.5 hours. The covered baking phase keeps moisture locked in while the oven slowly finishes tenderizing the meat. After 90 minutes, the brisket will be fully tender and the flavors will have intensified beautifully.

Remove the foil and raise the oven temperature to 400°F (200°C). Brush on another generous layer of glaze and bake uncovered for a further 20–25 minutes until the glaze is deeply caramelized and the surface is beautifully browned. Keep an eye on it during this phase — the sugar in the glaze can go from perfect to burnt quickly.