Preheat oven to 325°F.
Remove the meat and spice packet from the package.
Lay the meat, fat side up, on a large piece of heavy-duty aluminum foil (large enough to fold seals on top and 2 ends).
Evenly sprinkle peppercorns from package over the beef.
In a small bowl, whisk together honey mustard, whole grain mustard, and brown sugar until combined.
Spread half of the mustard mixture over the top of the meat.
Reserve remaining mustard to serve on the side
Wrap the corned beef with foil so that space is left on top in between the corned beef and the foil
Place foil-wrapped corned beef in a shallow roasting pan and bake for 2 hours.
Open the foil wrapping and bake another 30 minutes, until the top is lightly browned.
Let the meat rest for 10 to 15 minutes, then place on cutting board and cut at a diagonal, across the grain of the meat, into 1/2-inch thick slices.
Serve with reserved mustard on the side.