Freeze the flank steak for 30-50 minutes to make it easier to slice. Slice the meat against the grain into ¼ inch slices.
Place the cornstarch in a bowl and toss the meat pieces in the cornstarch. Let sit for 15 minutes.
Heat the one cup of vegetable oil in a large pan over medium-high heat.
Add the cornstarch coated beef to the hot oil. Be careful to not crowd the pan and cook in batches. Cook 1-2 minutes per side (it will cook more in the sauce later). Remove meat to a plate. Be careful of hot oil! Leave all those yummy bits of fry stuff left in the pan. They will add to the sauce flavor.
Now to make the sauce. Empty out all the oil from the pan except about 1 tablespoon. Reduce the heat to medium.
Add the ginger and garlic and stir for about one minute.
Add the brown sugar first because you don’t want to add liquid to hot grease. Stir. Then add the soy sauce, water, hoisin sauce, sriracha and red pepper flakes to the pan. Bring to a boil and simmer until it thickens. Stir occasionally scraping up the yummy bits at the bottom of the pan.
You can add a cornstarch/water mixture if it doesn’t get as thick as you like. I added 1 tablespoon cornstarch mixed with 1 tablespoon cold water.
Once the sauce has thickened add the meat back into the pan. Add the sesame oil, red pepper flakes, and green onions. Stir to combine and heat through.
Serve over rice and enjoy!