Cut off the root end of enoki mushrooms and rinse them under running water.
Slice the beef into bite-sized pieces.
Julienne the onion and green onion.
In a bowl, mix together soy sauce, sugar, minced garlic, sesame oil, and pepper to make the marinade.
Combine the beef and marinade and let it sit for 10 minutes.
Heat cooking oil in a pan and stir-fry the beef.
When the beef is halfway cooked, add the onion, green onion, and enoki mushrooms and stir-fry together.
Once all ingredients are cooked through, turn off the heat and serve on a plate.