Eight days before serving, combine salt, and brown sugar in a small bowl and whisk until homogenous. Rub evenly over every surface of brisket. Combine peppercorns, mustard, coriander, allspice, cloves, ginger, and bay leaves and sprinkle evenly over both sides of beef, pressing spices gently into the meat until they stick. Seal the beef in a vacuum sealed bag or a zipper-lock bag with all of the air pressed out of it. Place in coldest part of refrigerator and let rest for 7 days, flipping once a day.
The day before serving, remove the beef from the bag and carefully rinse off all spices under cold running water. Pat dry with paper towels. Bring a large pot of water up to 180°F, drop in the beef, cover the pot, then turn down the heat to the lowest possible setting. Adjust the heat as necessary so that the water hovers at around 180° for the entire cooking period. Cook until completely tender, about 10 hours.
Once beef is cooked, transfer to an airtight container along with cooking liquid Let rest at least overnight, or up to 3 days.
The day of: transfer cooking liquid to a large saucepan along with carrots, potatoes, and cabbage. Top with water until vegetables are submerged. Slice beef thinly against the grain and fan slices out in large skillet. Add 1 cup of liquid from pot to skillet and place skillet on top of pot. Cover skillet. Bring the pot to a boil over high heat, then reduce to a simmer. Cook until vegetables are completely tender and beef is heated through, about 45 minutes.
Serve immediately with hot mustard.
