In a large bowl, whisk together the soy sauce, garlic, sesame oil, sugar, ginger, and black pepper in a bowl. Add the steak and toss to coat with the marinade. Cover with plastic wrap and marinate in the refrigerator for a minimum of 24 hours.
Heat the oil in a wok or large skillet over high heat. Add the beef and green onions, and cook until the meat is no longer pink, stirring, 2 to 3 minutes. Add the mushrooms and mung beans and cook, stirring, until wilted and warmed through, 30 seconds to 1 minute.
Place the watercress in a large bowl and toss with the beef mixture and Kim Chee. Divide among 4 plates and garnish with the cilantro. Serve immediately.
Yield: 4 servings