In a large, shallow bowl, whisk together soy sauce, calamansi juice (or lemon/lime juice), black pepper, and minced garlic to form your marinade..
Place the beef on a clean, flat surface, and pound it out thinly with a meat mallet. Then, thinly slice the beef, and place the slices in the marinade.
Heat oil in a pan over medium-low heat, then add the onions and saute until translucent and slightly browned, about 10-15 minutes. Remove and set aside.
In the same pan, add the marinated beef. Cook until golden brown, about 2-3 minutes per side. Cook in batches to avoid overcrowding the pan.
Combine the onions and beef in the pan, then stir to combine and heat through, another 1-2 minutes.
Serve over steamed rice and enjoy!