Prep the beef: Trim excess fat and cut the chuck roast into 3–4 large pieces. This helps it cook evenly and shred easily later.

Stir the sauce: In a bowl, whisk together soy sauce, brown sugar, beef broth, rice vinegar, gochujang, gochugaru (if using), sesame oil, garlic, and ginger until smooth.

Layer the crock: Scatter the sliced onion in the bottom of the slow cooker. Place the beef on top. Pour the sauce over the beef, turning the pieces to coat.

Cook low and slow: Cover and cook on Low for 7–8 hours, or on High for 4–5 hours, until the beef is very tender and shreds with a fork. Low produces the best texture.

Shred the beef: Transfer the beef to a cutting board and shred with two forks. Skim any excess fat from the cooking liquid if needed.

Thicken the sauce (optional): Switch the slow cooker to High. Stir in the cornstarch slurry and cook for 10–15 minutes until the sauce slightly thickens and turns glossy.

Combine and finish: Return the shredded beef to the sauce and toss to coat. Taste and adjust: add a splash of soy for salt, a pinch of sugar for sweetness, or a little vinegar for brightness.

Serve: Spoon the beef over hot rice, tuck into lettuce wraps, or pile onto noodles. Garnish with green onions and sesame seeds.