How to Make It
Coat bottom and sides of a 6-quart slow cooker with cooking spray. Layer onion slices in bottom of slow cooker; top with garlic.
Heat canola oil in a large cast-iron skillet over medium-high heat. Add chuck roast to pan; cook 2 to 3 minutes, turning to brown on all sides. Place beef on top of onions and garlic; pour any drippings from pan over top.
Combine stock, soy sauce, 3 tablespoons vinegar, 1 tablespoon sesame oil, and next 4 ingredients (through red pepper) in a medium bowl, stirring with a whisk. Pour mixture over beef and onions in slow cooker. Cover and cook on LOW for 8 hours.
Place cucumber and carrot in 2 shallow bowls 15 minutes before meat is done. Combine remaining 1/2 cup vinegar, 1 cup water, and granulated sugar in a small saucepan over high heat; bring to a boil. Remove from heat; divide vinegar mixture evenly between cucumbers and carrots. Let stand until ready to serve; drain well.
Transfer beef from slow cooker to a large platter; shred with 2 forks. Heat remaining 1 tablespoon sesame oil in a large cast-iron skillet over medium-high heat; add shredded beef to pan, pressing into an even layer. Cook 2 to 3 minutes, without stirring. Turn off heat. Sprinkle evenly with salt and 1/4 cup cooking liquid from slow cooker; toss to coat. Discard remaining cooking liquid, onions, and garlic. Divide beef and pickles among lettuce leaves; sprinkle with sesame seeds.