BBQ Brisket Sandwiches with Quick Slaw

In a 5- to 6-quart slow cooker, whisk together the tomato sauce, sugar, mustard, chili powder, Worcestershire, and 2 tablespoons vinegar. Add the onion and garlic and toss to combine. Season the brisket with 1/2 teaspoon pepper, then cut crosswise into 2 1/2-inch pieces. Add the meat to the slow cooker and turn to coat. Cook, covered, until the meat is tender and easily pulls apart, 7 to 8 hours on low or 5 to 6 hours on high. Thirty minutes before the beef is done, in a large bowl, whisk together the sour cream, remaining tablespoon vinegar and 1/4 teaspoon salt. Cut the cabbage in half, core and thinly slice. Add to the bowl and toss to coat. Using 2 forks, shred the beef, then stir it into the cooking liquid and onions. Serve on the buns, topped with coleslaw, pickles, and chips, if desired.