Method

1. Mix soy sauce and beef slices and marinate for at least half an hour
2. In a saucepan with oil, saute onion rings for 3 minutes. Transfer to a bowl. Set aside. Drain marinated beef and save marinade.
3. In the same pan where you sauteed the onions, saute beef slices until brown on both sides. Transfer to a bowl. Set aside.
4. Pour marinade in the same pan and bring to a boil. When it boils, simmer for 5 minutes. If sauce is drying up add water.
5. Put back the beef and toss together for 3 minutes. Put back onion rings a couple of minutes before turning.