In a skillet over medium-high heat, brown the ground beef. Halfway through, stir in the minced garlic. Once cooked through, drain any excess grease and transfer the beef and garlic to your slow cooker.
In a small bowl, whisk together soy sauce, hoisin sauce, sesame oil, ginger, black pepper, red pepper flakes (if using), brown sugar, and beef broth. Pour this sauce over the beef in the slow cooker.
Add the sliced onions and stir well to combine.
Cover and cook on low for 4–5 hours or high for 2–3 hours.
About 30 minutes before serving, stir in the coleslaw mix. This allows it to soften slightly while still keeping some crunch.
Just before serving, add the cooked chow mein noodles and toss until evenly coated in the sauce.
Garnish with sesame seeds and green onions, then serve hot.
