Filipino Bistek (Beef with Citrus Soy Sauce)

My ex-boyfriend's mother used to always make this when she got off work late and had to put a meal on the table in a rush. It is so comforting and goes excellent on top of some steamed white jasmine rice.

Ingredients:
2 lbs beef flank steak, sliced into 1/4 inch thick pieces
1/2 cup of soy sauce
1 lemon, juiced (or if available, used kalamansi)
pepper, to taste
1 large onion, sliced

Method:
1. Marinade the beef with the soy sauce, lemon juice, and pepper for approximately 30 minutes.
2. Heat a wok with some oil over high heat.
3. Once the oil begins to smoke, throw in the sliced onions and stir for approximately 30 seconds.
4. Pour the beef with the marinade into the wok and stir to cook for approximately 3-4 minutes or until the beef is cooked and the onions are tender.
5. Serve over steamed white rice.