Slice the beef into thin strips.
Prepare the vegetables: chop the onion, capsicum, and green beans.
Mince the garlic.
Make the sauce by combining the oyster sauce, soy sauce, water, corn starch, and sesame oil.
In a pan or wok, stir fry the beef with the onion, capsicum, green beans, garlic, black pepper, brown sugar, and salt.
Add the sauce and cook until the sauce thickens.
Serve the beef stir fry hot.
