To cook en papillote (in the oven, enclosed in an oven bag or foil), preheat oven to 325 degrees F. Place flour in an oven bag and shake to coat the interior. Add cracked pepper and brisket to bag and shake to coat surface of meat with pepper. Place bagged brisket in roasting pan. Add honey mustard, horseradish and 1/2 cup of water and seal bag. Bake for about 45 minutes per pound.
On the episode we cooked this dish sous vide, French for "under vacuum," and which applies to cooking vacuum packed foods at a very low temperature of around 140 degrees F usually in liquid. Because of food safety concerns, special equipment is required for true sous vide cooking. If you happen to own one of the appliances which vacuum seals food in plastic bags which are safe for boiling, you may wish to use their "boil-in-bag" method, which is similar to sous vide. However, be sure to follow the manufacturer's instructions.
Alternatively you may wish to use a "faux sous vide" cooking method (done similarly to sous vide, only without a vacuum). To do this you would wrap the ingredients in plastic and immerse them in a court bouillon - the term for a "poaching liquid" consisting of a vegetable broth - and simmer on the stovetop at a low temperature (necessary to avoid melting the plastic) for about 2 hours. (A "simmer" in classical cooking is just below a boil.)
While the corned beef is cooking, prepare the honey mustard sauce:
Combine the milk, onion, bay leaf, cloves and nutmeg in a heavy-gauge, non-corrosive saucepan over medium heat. In a separate pan create a roux by cooking the butter and flour for a few minutes, stirring continuously, without browning, until it emits a nutty aroma. Remove from the heat. When the milk is fairly hot, pour a small amount of it into the cooled down roux, stirring until the milk is thoroughly blended in. Return this to the remaining milk, and simmer for 15 minutes, stirring frequently. Remove from heat and season, to taste, with salt and white pepper. Whisk in honey mustard and cover with a piece of plastic wrap to prevent a skin from forming.
Let corned beef rest 10 minutes before carving. Serve slices on open face rye with honey mustard sauce on the side.