Make the sauce. Heat 1 tbsp vegetable oil in a medium saucepan over medium heat. Add ginger and garlic and stir for 30 seconds.
Add soy sauce, water, and brown sugar. Bring to a boil and simmer until thickened some, 10 to 15 minutes.
Slice flank steak against the grain into ¼-inch slices with the knife held at a 45 degree angle. Some of the really long pieces I cut in half to make them more bite-sized.
Toss flank steak with cornstarch and let sit for 10-15 minutes.
Heat 1 cup vegetable oil in a large pan over medium-high heat.
Add beef (I cook in two batches) and cook 2-3 minutes, until brown and crispy, flipping pieces over to cook both sides.
Remove meat with a slotted spoon and discard vegetable oil. Place meat back in pan along with sauce and cook over medium heat for 1 minute, stirring to coat meat.
Add red pepper flakes and green onions and remove from heat.
Serve over rice.