1 cup beef broth
1 jar (8 ounces) hoisin sauce
¼ cup white vinegar
2 teaspoons Chinese five-spice powder
2 teaspoons garlic powder
2 ribs celery, cut into 2-inch long matchsticks
1 thin-cut beef brisket (2 pounds) well trimmed of excess fat
½ teaspoon black pepper
1 head bok choy, trimmed and thinly sliced
8 soft sandwich buns
Whisk beef broth, hoisin sauce, ½ cup water, vinegar, five-spice powder and garlic powder in a small bowl. Place celery and carrot strips in slow cooker bowl. Add the brisket (slicing in half to fit, if necessary); season on all sides with salt and pepper. Pour in beef broth mixture. Cover; cook for 5 hours on HIGH or 10 hours on LOW.
Remove meat to a cutting board. Cool slightly. Skim any fat off top of sauce in cooker and discard. Add bok choy to slow cooker and cook for 15 minutes on high.
Meanwhile, shred beef with two forks, cutting long pieces in half, if desired. Add back to slow cooker and stir to coat with sauce. Serve immediately with soft sandwich buns.