Place noodles in bowl. Cover with boiling water. Let stand 15 minutes. Meanwhile, bring stock to boil in medium saucepan. Reduce heat and keep warm. Heat vegetable oil in heavy large skillet or wok over high heat. Add beef, half of green onions and 3 tablespoons ginger and stir-fry until beef is no longer pink, about 4 minutes. Add bok choy and stir-fry 1 minute. Add stock, remaining green onions, remaining 2 tablespoons ginger, 2 tablespoons soy sauce, 1 tablespoon sesame oil and pepper and bring to simmer. Drain noodles and divide among bowls. Ladle soup over. Serve, passing additional sesame oil and soy sauce. *Also known as transparent or cellophane noodles. Available at oriental markets and many supermarkets. soup 50