Clean and prep the bean sprouts.
Slice the beef thinly.
Julienne the onion and green onion.
Heat cooking oil in a pan and stir-fry the beef.
When the beef is partially cooked, add the onion, green onion, and bean sprouts, and stir-fry.
Season with soy sauce, minced garlic, red pepper powder, and black pepper, and mix well.
When all ingredients are cooked through, serve on a plate.