Wrap the steak in plastic wrap and place in the freezer for 30 minutes. Unwrap and slice across the grain into 1/4-inch thick slices. In a large bowl, toss the beef with the cornstarch to coat.
Make the sauce: in a small saucepan over medium heat, combine the water, brown sugar, soy sauce, garlic, ginger and oil. Bring to a boil, then reduce heat and simmer, stirring occasionally, until slightly thickened, about 5-10 minutes.
Heat the vegetable oil in a large cast iron skillet. Working in batches so the pan isn’t crowded, add the beef and fry until browned and cooked through, about 1-2 minutes. Transfer to a paper towel-lined plate and discard the excess oil.
Return the beef to the skillet, add the sauce, and cook over medium heat, stirring occasionally, until the sauce thickens, about 2-3 minutes. Stir in the green onions.
Serve immediately over rice, garnished with sesame seeds.
Freezing the steak briefly makes it much easier to slice thin, even pieces. The cornstarch both tenderizes the beef and thickens the sauce, so don’t skip it. Use reduced sodium soy sauce to keep the dish from getting too salty.