Step 1

Trim fat from brisket. Cut brisket to fit into a 3-1/2- or 4-quart slow cooker (see Tip). Place onion in the slow cooker. In a small bowl, combine chili powder, garlic powder, celery seeds, and pepper. Sprinkle chili powder mixture evenly over meat; rub in with your fingers. Place meat in the slow cooker over onion.

Step 2

In a medium bowl, combine ketchup, chili sauce, brown sugar, vinegar, Worcestershire sauce, liquid smoke, if using, and dry mustard. Pour over brisket in cooker.

Step 3

Cover and cook on low-heat setting for 9 to 10 hours or on high-heat setting for 4-1/2 to 5 hours.

Step 4

Remove brisket from cooker to a cutting board. Remove onion from cooker with a slotted spoon to a small bowl, reserving cooking juices. Skim fat off cooking juices; measure 3/4 cup of the cooking juices. For sauce: In a small saucepan, stir the cold water into flour; add the 3/4 cup cooking juices. Cook and stir over medium heat until thickened and bubbly; cook and stir for 1 minute more.

Step 5

Thinly slice brisket across the grain. Place about 3 ounces cooked meat atop each bun bottom. Top with onions and drizzle each with about 1-1/2 tablespoons sauce to serve.