Slice the beef thinly. For easy slicing, partially freeze the beef for 20–30 minutes, then cut against the grain into thin strips. Season lightly with salt and pepper.
Make the sauce. In a bowl, whisk together beef broth, soy sauce, brown sugar, rice vinegar, sesame oil, garlic, ginger, and red pepper flakes.
Load the slow cooker. Add the sliced beef and onions to the crock pot.
Pour the sauce over the top and stir to coat evenly.
Cook low and slow. Cover and cook on Low for 4–5 hours, or on High for 2–3 hours, until the beef is tender but not falling apart.
Thicken the sauce. In a small bowl, mix cornstarch with cold water to make a slurry. Stir it into the crock pot. Cook on High for 15–20 minutes until the sauce thickens and becomes glossy.
Add the broccoli. Stir in the broccoli florets.
Cook for 15–25 minutes on High, until broccoli is crisp-tender. If using frozen broccoli, thaw and pat dry first, then cook for 10–15 minutes.
Finish and serve. Taste and adjust seasoning. Garnish with sesame seeds and green onions.
Serve hot over rice or noodles.