Slice steak at a 45 degree angle across the grain, slicing almost, but not fully through. Make another slice 1/4 inch from the first one, following the same angle and slicing all the way through. Open up butterflied pieces and lay in a large bowl.
Whisk together soy sauce, green onions, sesame oil, sugar, and garlic. Pour over beef, coating fully. Wrap with plastic wrap and place in refrigerator for at least 30 minutes and up to 4 hours.
Preheat grill to high heat. Lay beef opened up on grill and cook 2 to 3 minutes per side, flipping once with the lid down.