Preheat your oven to 300°F (150°C).

Rinse the corned beef brisket under cold water to remove any excess salt from the brine. Pat it dry with paper towels.

In a small bowl, mix together the Dijon mustard, whole grain mustard, and honey to create a mustard glaze.

Place the corned beef brisket in a roasting pan or Dutch oven, fat side up.

Spread the mustard glaze evenly over the surface of the brisket, coating it completely.

Scatter the onion quarters, smashed garlic cloves, and bay leaves around the brisket in the roasting pan.

If desired, sprinkle the seasoning packet that came with the corned beef over the brisket. You can also add additional spices like peppercorns or cloves for extra flavor.

Pour enough water or beef broth into the roasting pan to almost cover the brisket. The liquid will help tenderize the meat and keep it moist during roasting.

Cover the roasting pan with a lid or aluminum foil and place it in the preheated oven.

Roast the corned beef brisket for about 3-4 hours, or until it is fork-tender. Check for doneness by inserting a fork into the meat—it should slide in easily with little resistance.

Once the corned beef is cooked, remove it from the oven and let it rest for about 10-15 minutes before slicing. This allows the juices to redistribute and the meat to firm up slightly for easier slicing.

Slice the roasted corned beef brisket against the grain into thin slices. Serve it with your favorite side dishes such as roasted potatoes, steamed cabbage, or glazed carrots.

Enjoy your delicious roasted corned beef with mustard as a flavorful and comforting meal!