Slice the beef into 1/2 inch strips against the grain.
Place the in slow cooker along with the onion, beef broth, soy sauce, brown sugar, oil, vinegar, garlic and ginger . Give it a quick a stir.
Place on high heat for 3 hours.
Add the frozen broccoli. Spoon out 1/2 cup of liquid and whisk with the cornstarch, return to the sauce to thicken. Continue to cook for 15 minutes on low heat.
Serve over preferred rice and garnish with sesame seeds and scallions, if desired.
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Notes
If using fresh broccoli, cooking time might increase to 30 minutes.