Combine white miso paste, reduced-sodium soy sauce, ginger paste, granulated sugar, mirin, seasme oil, chopped garlic, and Kosher salt in a medium bowl, then whisk.
Add steak bites, then stir well to evenly coat. Let sit at room temperature.
Heat a large skillet over high heat. When very hot, add marinated steak in a single layer, then cook, stirring occasionally, or until browned on all sides, but medium rare in the center. If you prefer steak closer to medium or medium well, cook.
Either transfer the steak to a serving tray or serve directly in the skillet. Sprinkle with thinly sliced scallions and lots of fresh cilantro. Serve immediately.