Cut meat into 1/2 cm (1/4 inch) slices against the grain. Proceed to prep the meat with either version 1 (short) or version 2 (overnight)
Add salt, pepper, sesame oil, and corn starch to a zip-lock bag. Add sliced beef, close and move the meat around so it gets evenly coated.
Add salt, pepper and sesame oil to a zip-lock bag. Add sliced beef and sliced onions, close and move the meat around so it gets evenly coated. Leave int he fridge overnight to marinate.
After the meat is coated or marinated it's ready to cook. If you've used the marinating method, remove onions from the meat before cooking the meat, set onions aside, and cover the meat with corn starch.
Heat just enough oil to cover the bottom of a wok or a pan over high heat just until it starts smoking. Add meat to the pan evenly, but don't overcrowd. If necessary fry in batches. Fry for a minute on one side, then turn around and fry for another 30 seconds. Remove from the pan and set aside.
In a bowl mix together 1 cup water, soy sauce, oyster sauce, and rice vinegar.
Add garlic and ginger and fry for a minute or until golden.
Add sliced onions. If you've used the marinating version, you can use the onions that you've used to marinate the meat or fresh onion. Fry just until golden.
Add the soy sauce mixture and brown sugar and stir well.
In a separate bowl mix together 2 TBSP water and 1 TBSP corn starch. Add to the sauce and stir frequently for about a minute or two. This will thicken the sauce.
Add meat and broccoli and stir well.
Serve with steamed rice.