Bring soy sauce, mirin, vinegar, ginger, garlic, and red-pepper flakes to a boil in a saucepan on high heat. Reduce heat; simmer until reduced by half, about 5 minutes.
Heat a cast-iron skillet over high heat; add oil. Season steak all over with salt and pepper. Cook for 3 minutes on each side. Reduce heat to medium, and brush top of steak with some glaze; flip, and cook for 1 minute. Brush with remaining glaze, flip, and cook for 1 minute more. Transfer steak to a cutting board; let rest for 5 minutes. Slice steak against the grain into 1/8-inch-thick strips, and serve on top of or alongside spring green salad.