Toss beef with cornstarch, dark soy sauce, soy sauce, and rice vinegar. Set aside to marinate.
In another bowl, mix black pepper, dark soy sauce, soy sauce, rice vinegar, and 2 tablespoons water. Set aside.
Heat 1 tablespoon oil in a skillet or wok over medium-high heat.
Cook beef in a single layer until browned, about 2 minutes per side.
Transfer beef to the dressing bowl and toss to coat. Wipe out the skillet.
Heat remaining oil in the skillet over high heat.
Stir-fry onion until fragrant, about 30 seconds.
Add peppers, ginger, and garlic. Cook until softened and starting to char, about 5 minutes.
Reduce heat to medium. Return beef and dressing to the skillet. Toss to combine.
Stir in a little water if needed to loosen the sauce. Season with kosher salt to taste.
Serve with steamed white rice.