Prep the beef: Trim excess fat and cut the chuck roast into 3–4 large chunks. This helps it cook evenly and shred easily.
Make the sauce: In a bowl, whisk together honey, soy sauce, beef broth, minced garlic, grated ginger, rice vinegar, sesame oil, black pepper, and red pepper flakes if using.
Load the crockpot: Place the beef into the slow cooker. Pour the sauce over the top, turning the pieces to coat.
Set and cook: Cover and cook on Low for 7–8 hours or High for 4–5 hours, until the beef is fork-tender.
Shred the beef: Remove the beef to a cutting board and shred with two forks.
Skim excess fat from the surface of the sauce if needed.
Thicken the sauce (optional): Whisk the cornstarch slurry into the crockpot. Turn to High and cook for 10–15 minutes until thick and glossy.
Combine and finish: Return the shredded beef to the slow cooker, toss to coat, and let it sit for 5 minutes to absorb the sauce.
Serve: Spoon over rice, noodles, or mashed potatoes. Garnish with sesame seeds and green onions.