Toss rib-eye cubes with cornstarch, dark soy sauce, soy sauce, and rice vinegar. Marinate.
Stir together black pepper, dark soy sauce, soy sauce, rice vinegar, and 2 tablespoons water. Set aside.
Heat 1 tablespoon oil in a large skillet over medium-high heat. Add beef in a single layer.
Cook until browned, 2 minutes per side. Transfer to dressing bowl and toss. Wipe out skillet.
Heat remaining oil over high heat. Stir in onion until fragrant, 30 seconds.
Add peppers, ginger, and garlic. Cook until softened and charred, about 5 minutes.
Reduce heat to medium. Return steak and dressing to skillet. Toss to combine.
Add water if needed. Season with salt to taste.
Serve with steamed white rice.