How to Make It
Season the brisket all over with salt and let stand at room temperature for at least 30 minutes or for up to 4 hours.
Preheat the oven to 325°. Set a large roasting pan over 2 burners and add all of the remaining brisket ingredients. Nestle the brisket in the braising liquid and bring to a boil over high heat. Cover with foil and braise in the oven for about 3 hours, or until the brisket is very tender. Let the brisket cool in the braising liquid, then cover and refrigerate overnight.
Preheat the oven to 300°. In a food processor, process the rye bread to coarse crumbs. Add the butter and sugar; pulse until evenly coated. Spread the crumbs on a large rimmed baking sheet and bake for about 20 minutes, stirring occasionally, until they start to dry out. Increase the oven temperature to 350°, season the crumbs with salt and bake for about 12 minutes longer, stirring occasionally, until golden brown and crisp. Let cool completely.
Skim the fat from the surface of the braising liquid and let the brisket return to room temperature. Lower the oven temperature to 325°. Cover the brisket and bake for about 1 hour, or until heated through. Transfer the brisket to a carving board and cover with foil. Strain the braising liquid into a medium saucepan, reserving the vegetables. Skim the fat from the braising liquid and boil over high heat until reduced to 6 cups, about 20 minutes. Season with salt and pepper.
Increase the oven temperature to 350°. Trim any fat from the surface of the brisket and cut it into 12 pieces. Transfer the brisket pieces back to the roasting pan. Pour in enough of the hot braising liquid to reach halfway up the meat. Spread 1/2 tablespoon of mustard over the top of each piece. Return the vegetables to the roasting pan. Cover the brisket with foil and bake for about 15 minutes, or until heated through.
To serve, pour 1/2 cup of the hot braising liquid into each of 12 shallow serving bowls and add some of the braised vegetables. Place a piece of brisket in each bowl, top with 1/4 cup of the crisp rye bread crumbs and serve.