Slice steak into very thin strips (1/4 inch or less) against the grain. Tip: you can put it in the freezer for 15 minutes first to make it easier to cut.
Place half of the cornstarch into a ziptop bag, close and shake to coat sides of bag.
Add steak to the bag, then sprinkle the remaining cornstarch on top.
Make sure plenty of air is inside of the bag, then zip it closed.
Shake the bag to coat steak in cornstarch.
Let steak sit for 10 to 15 minutes so that cornstarch adheres well.
In the meantime, make the sauce by adding the soy sauce, water, ginger, garlic and brown sugar to a medium-sized sauce pan.
Whisk well and cook on medium heat at a bubble for 8 to 10 minutes.
Sauce volume will be reduced by about half. Remove from heat.
In a hot wok, fry steak in 1/2 cup avocado oil, about 2 to 3 minutes per batch.
Remove steak and place on a plate with paper towels.
Return sauce to the stove and warm to medium heat until bubbling.
Add green onions and cook 1 to 2 minutes.
Add cooked steak to pan with sauce, stir to coat.
Serve over your favorite rice. Enjoy!