Heat 2 teaspoons oil in a small pan over medium low. Add ginger and garlic and stir just until fragrant (about 1 minute). Add soy sauce, water and brown sugar and bring to a boil. Let boil 3-5 minutes or until slightly thickened. Set aside.
Slice the flank steak into ¼" slices and toss with cornstarch. Gently shake off any excess.
Place 1 tablespoon at a time of oil in a pan or wok and heat over medium high heat. Cook the beef in small batches for about 2 minutes. (It doesn't need to cook all of the way through, it will cook completely when combined with the sauce).
Once all the beef has been cooked, combine with the sauce and heat over medium until hot and bubbly. Remove from heat and stir in green onions. Serve over rice. Chopsticks optional. :)