01 - Whisk together soy sauce, brown sugar, water, hoisin sauce, sesame oil, garlic, ginger, and red pepper flakes in a medium bowl until sugar dissolves completely.
02 - Place sliced flank steak in a large zip-top bag, add cornstarch, seal tightly, and shake vigorously to coat all pieces evenly with starch.
03 - Transfer coated beef to slow cooker, pour sauce over meat, and stir to distribute. Cover and cook on low setting for 4 hours until beef reaches fork-tender consistency.
04 - During final 30 minutes, whisk cornstarch with cold water in small bowl until smooth, then stir into slow cooker. Continue cooking until sauce thickens to coating consistency.
05 - Serve immediately over steamed rice or noodles, topped with sliced green onions and toasted sesame seeds.