Cut beef and mushrooms into bite-sized pieces.
Slice onion and green onion thinly.
Heat cooking oil in a pan and stir-fry the beef.
When the beef is halfway cooked, add mushrooms, onion, and green onion. Continue stir-frying.
Season with soy sauce, sugar, minced garlic, and pepper.
Once all ingredients are cooked, turn off the heat and stir in sesame oil.
Transfer to a plate and serve.