Rub cracked pepper, salt, and garlic all over brisket. Use the tip of a sharp knife to cut ½-inch slits across the top of the roast, into the fat cap. Place the roast into the slow cooker.
In a measuring cup, or medium-sized bowl, whisk together the soy sauce, balsamic vinegar, Worcestershire sauce, and ground dried mustard, until mustard has dissolved. Pour over the roast.
Cover and cook on LOW for 8 to 10 hours.
Use tongs and/or spatula to carefully remove the cooked brisket to a cutting board. Loosely tent with foil for about 10 minutes.
Strain the sauce from the slow cooker through a sieve into a fat separator, or a medium-sized bowl. If using the fat separator, pour the liquid into a medium-sized saucepan until only fat remains. If not using a fat separator, use a spoon to skim the fat that is has risen to the top of the liquid. Discard grease. Transfer liquid to saucepan.
Bring the sauce to a boil over high heat. Reduce heat to medium and simmer until sauce has thickened slightly.
Slice the brisket into ½-inch slices and place on a platter. Drizzle some of the sauce over the meat slices. Serve at once, passing sauce at the table.