In a large zip lock bag, place the sliced beef inside and add the first 6 ingredients. Marinade for 8 hours or overnight.
In a large stainless steel sauté pan, heat up the avocado oil on medium heat and add the onions. Cook until the onions are soft and transparent. Set the onions aside.
Add more avocado oil and add the beef. Make sure not to overcrowd the pan.
Fry the beef on both sides and set it on a large serving platter.
In a small bowl, mix the corn starch in 1 cup of water and set it aside.
Add the remaining marinade left over in the zip lock bag to the pan and add the cornstarch and water mixture. This will help thicken up the sauce. Let it come to a boil and then lower the heat to a simmer for about 5 minutes.
Pour the sauce over the beef, then add the sliced onions.
(OPTIONAL) Sprinkle some green onions on top.
Serve with white rice and a side of sliced tomatoes.