In a mixing bowl, combine soy sauce, brown sugar, sesame oil, rice vinegar, minced garlic, grated ginger, and gochujang. Stir until sugar is dissolved.
Prepare the chuck roast by trimming excess fat and cutting into 2-inch chunks.
Place beef in the slow cooker and pour the marinade over it. Toss to coat.
Cook on low setting for 6-8 hours until beef is tender.
Add green onions and stir to incorporate just before serving, garnish with sesame seeds.
Notes
Store leftovers in an airtight container in the fridge for up to 3 days, or freeze for up to 3 months.