Mix together soy sauce, brown sugar, ginger, pepper flakes, one tablespoon of cornstarch and beef broth. Set it aside.
In a separate bowl, mix two tablespoons of cornstarch, water and garlic. Add the sliced beef and stir to coat.
Heat two tablespoons of oil in a frying pan over medium high heat and stir fry the coated beef for 3-4 minutes until cooked through. Remove the beef from the frying pan and set aside.
Add the broccoli and onions to the frying pan and stir fry for 3-5 minutes, until tender.
Return the beef to the pan. Add the soy sauce mixture. Cook and stir for 3-5 minutes, until thickened.