Cut the flank steak across the grain into thin strips about 1/4-inch thick. Add the dark soy sauce, brown rice wine and sesame oil. Marinate the steak for 15 minutes.While the steak is marinating, prepare the vegetables: rinse the bean sprout in warm running water and drain. Chop onions. Smash, peel, and chop the garlic cloves. If using refrigerated rice noodles, rinse them in warm water to loosen. Cut the noodles into 1/2 to 3/4 inch strips. Heat the wok and add 2 tablespoons oil. When the oil is hot, add the beef. Lay flat and let brown for a minute. Stir in the black bean sauce and stir-fry the beef until it is browned and nearly cooked through. Remove from the wok. Clean out the wok and add 2 more tablespoons oil. When the oil is hot, add the garlic and stir-fry until aromatic. Add the rice noodles and stir fry until browned. Push the rice noodles up to the side of the wok and add 1 more tablespoon oil. When the oil is hot, add the onion and bean sprout. Stir-fry briefly, then mix with the noodles. Stir in the oyster sauce. Add the beef back into the wok. Mix everything together and serve hot.