Slice the steak across the grain into thin strips about 1⁄4-inch wide and 2-inches long. You’ll find it easier to thinly slice if it is slightly frozen.
In a large bowl, mix the beef with the soy sauce, corn starch, wine and sugar, set aside.
In a large frying pan, heat 2 tablespoons of the oil over medium-high heat.
Add the mushrooms.
Sauté until the mushrooms become soft (about 2 minutes).
Remove mushrooms from the pan, set aside on a plate.
Heat the remaining oil in the same frying pan, over medium-high heat.
Add the onion slices.
Stir in the beef mixture.
Continue sauteing until the meat is no longer red and the sauce thickens a bit.