Combine the beef broth, brown sugar, soy sauce, garlic, rice vinegar, sesame oil, onion powder, black pepper, and red pepper flakes in a large bowl. Set aside about ½ cup of the sauce and keep it in the refrigerator for later.

Add the sliced beef to the bottom of the slow cooker. Pour the sauce mixture (besides the reserved ½ cup) over the beef.

Cook on low for 3-4 hours or just until the beef is cooked through.

Whisk the reserved sauce and cornstarch together to make a slurry. Add the slurry to the slow cooker and gently stir to combine with the sauce.

Add the broccoli florets to the slow cooker. Do not mix so the broccoli can steam in the slow cooker..

Cook for an additional 30 minutes - 1 hour until the sauce has thickened and the broccoli reaches your preferred tenderness. The beef should be cooked through and tender when done.

Gently fold in the steamed broccoli to coat it with the sauce. Serve over rice and garnish with sesame seeds if desired.