Combine all marinade ingredients in a bowl, then marinade the beef for about 30 minutes to an hour.
Squeeze the meat with your hands to remove excess marinade from the meat. Set the marinade aside for later use.
Heat oil over medium high heat. Add the onion rings and cook per your liking. I prefer mine not too cooked. Remove from oil and set aside.
On the same pan, fry the marinated beef until golden brown (about 1.5-2 minutes). Pour the soy sauce marinade then add the cornstarch dilluted in 1 cup water. Bring to a boil then reduce heat to lmedium low.
Simmer while stiring constantly and cook until the sauce thickens.
Top with onions and parsley. Serve hot with rice or mashed potatoes.