Make marinade: Combine soy sauce, sugar, mirin, sesame oil, oyster sauce, minced garlic, and pear juice. Mix until sugar dissolves.
Marinate: Combine beef, onion, leeks, and mushrooms in a bowl. Pour marinade over everything and mix well. Marinate at least 15 minutes — overnight preferred.
Soak noodles: Place glass noodles in a bowl and pour over boiling water. Soak 10 minutes until softened. Drain and set aside.
Assemble: To each earthenware pot add 200 ml water, half the beef and vegetable mixture, and a portion of drained glass noodles.
Cook: Bring to a boil over medium-high heat and cook for 15 minutes until beef is cooked through, noodles are silky and saturated, and broth has reduced into a rich concentrated stew.
Serve: Top with sliced green onion and serve immediately in the pot while bubbling hot alongside steamed white rice.