1. Place the sealed brined corned beef in a six-to-eight-quart slow cooker and cover with water. Set to low and cook for 10 to 12 hours. Allow to cool and refrigerate in the bag for at least one hour. This step is crucial for slicing.
2. Drain the liquid from the bag and slice corned beef into half-inch slices. Place in an ovenproof baking dish.
3. Preheat oven to 400 degrees Fahrenheit. Mix duck sauce and mustard. Pour over the corned beef and cover tightly with aluminum foil. Bake for 45 minutes, then remove foil and bake an additional 15 minutes to allow the glaze to darken and caramelize.