Whisk together 2 tbsp cornstarch with 3 tbsp water in a large bowl. Add the beef to the bowl and toss to combine.
In a separate bowl, whisk together the remaining 1 tbsp cornstarch with the soy sauce, brown sugar, garlic and ginger. Set the sauce aside.
Heat a large nonstick pan over medium heat. Add 1 tbsp of the vegetable oil and once hot, add the beef and cook, stirring constantly until the beef is almost cooked through. Transfer the beef to a plate and set it aside.
Add the remaining 1 tbsp of vegetable oil to the pan and once hot, add the broccoli florets and sliced onions and cook, stirring occasionally until the broccoli is tender, about 4 minutes.
Return the beef to the pan then add the prepared sauce. Bring the mixture to a boil and cook, stirring for 1 minute or until the sauce thickens slightly. Best served with rice or noodles.