In a large nonstick skillet over medium-high heat, add vegetable oil.
Once hot, add the marinated steak in a single layer, working in batches if needed. Cook for 2 to 3 minutes per side, until crispy. Transfer to a plate and tent to keep warm.
Carefully discard excess oil, leaving about 2 to 3 tablespoons in the skillet.
Add the sliced onion and cook for 2 to 3 minutes until slightly softened.
Reduce heat to medium. Add garlic and ginger and cook for 1 minute until fragrant.
Return the beef to the skillet. Pour in the prepared sauce and stir to coat.
Cook for 3 to 5 minutes, until the sauce is hot, slightly thickened, and the beef is heated through.
Garnish with chopped green onions and serve warm.