Preheat the oven to 375 degrees.
Layer a large piece of tin foil in a baking pan then place the corned beef fat side up on top of the foil.
Brush a thick coating of Dijon mustard over the entire piece of beef. Place fat side up on a large piece of tin foil.
Next, sprinkle the brown sugar on top evenly.
Wrap the corned beef with foil so that space is left in between the top of the corned beef and the tin foil.
Wrap in a second piece of foil. Place into the oven and bake for 3 hours.
Open the foil and broil the corned beef for 2-3 minutes, until the top is bubbly and lightly browned.
Watch carefully so it doesn’t burn.
Let the corned beef rest for at least 10 minutes before slicing across the grain of the meat.
Serve with the reserved extra Dijon mustard on the side if desired. Enjoy!