Sprinkle the steak with sugar, toss to coat and let sit for 20 minutes.
Add Japanese soy sauce, oil and cornflour to the steak and stir to mix.
In a large frying pan, heat 1 tablespoon oil over high heat. Stir-fry until medium, about 3-5 minutes. The cooking time will be dependent on the thickness of the steak and how you prefer your steak. Set aside.
Heat the same frying pan with butter and saute the garlic until fragrant, about 2-3 minutes. Add the steak and salt to taste, stir well and serve.