And for the sauce:
2 tsp veg oil
1/2 tsp. minced ginger
1 tbsp. chopped garlic
1/2 c. soy sauce
1/2 c. water
1/2 c. dark brown sugar

The directions for this recipe were mind-numbingly long, so let me give you the abridged version.

Start the sauce by heating 2 tsp veg oil in a saucepan over med/low heat. Don't get the oil too hot or you'll get a major splatter when adding liquids. Add ginger and garlic to the pan and quickly, then add the soy sauce and water immediately so that the garlic doesn't scorch.
Dissolve the brown sugar into the mixture, then raise the heat to medium and boil for 2-3 minutes or until thickened. Remove from heat.

Slice the flank steak against the grain into thing, about 1/4" slices.
Dip the steak pieces into the cornstarch to apply a very thin dusting to both sides of each piece. Let the beef sit for about 10 minutes so that the cornstarch sticks. As the beef sits, heat up one cup of oil in a wok or skillet.
Once the pan is hot, add the beef to the oil and saute for just 2 min. or until the beef just begins to darken on the edges. Stir the meat around a little so that it cooks evenly. After a couple minutes, use a slotted spoon to transfer the steak onto paper towels, then pour the oil out of the wok. Put the pan back over the heat, place the steak back in along with all of the onions and let it simmer for 1-2 minutes. Add the sauce, stir then cook for 2-3 more minutes.