Season both sides of the roast with salt and pepper. Place the roast into the slow cooker; add onions and garlic.
In a medium bowl, whisk together brown sugar, soy sauce, chili paste, ginger root, rice vinegar, and sesame oil until combined. Pour over roast and veggies.
Cook on HIGH for 4-5 hours or on LOW for 7-8 hours. Shred meat with two forks and gently stir back into the cooking liquid.
Serve Korean Beef over a bowl of sticky rice, wrapped in corn tortillas or lettuce leaves, and add your favorite toppings: Thinly sliced carrots and cucumber, chopped cilantro and green onions, or coleslaw mix tossed with 1 tablespoon soy sauce and 2 tablespoons rice vinegar, season with salt and pepper to taste. Pass the chili paste around the table for more added heat if desired, Enjoy!