01 - In a medium bowl, whisk together soy sauce, brown sugar, water, hoisin sauce, sesame oil, garlic, ginger, and red pepper flakes if using.
02 - Place sliced flank steak in a large zip-top bag. Add cornstarch, seal, and shake to coat the beef evenly.
03 - Transfer the coated beef to the slow cooker. Pour the sauce over the beef and stir to combine.
04 - Cover and cook on low for 4 hours, or until the beef is tender.
05 - In a small bowl, mix 1/4 cup cornstarch with 1/4 cup cold water to create a slurry. Stir the slurry into the slow cooker during the last 30 minutes of cooking to thicken the sauce.
06 - Serve hot, garnished with green onions and sesame seeds. Pair with steamed rice or noodles.