Place the beef in a bowl, and pour in the cooking wine, pear juice, and black pepper. Stir to combine, and allow to marinate for 30 minutes. Stir in the soy sauce, sugar, sesame oil, garlic, green onion, and sesame seeds, and marinate in refrigerator at least 2 hours or overnight.
Preheat an outdoor grill for medium-high heat.
Remove the beef from the marinade, and discard the marinade. Place a sheet of aluminum foil on the heated grill, and lay the beef slices separately on the foil. Place the mushrooms and onion on another part of the foil. Cook the beef slices until they are evenly brown, 3 to 5 minutes per side. Serve with cooked mushrooms and onion.
You can substitute 2 shredded fresh pears fro the pear juice.
For better taste, soak beef in cold water for a couple of hours. Change water occasionally.
OPTIONAL: Serve with a handful of Korean sweet-potato noodles. If pan-frying the beef, you can add 1/2 cup of beef broth to make a sauce to go over rice.