Freeze beef for 30 minutes until firm, then slice ⅛ inch thin against the grain. In a bowl, mix soy sauce, sugar, sesame oil, garlic, ginger (if using), black pepper, Asian pear (if using), and sliced onion. Add beef, massage the marinade into the meat, and let sit for 30 minutes at room temperature or up to 12 hours refrigerated.

Heat a large non-stick skillet or cast-iron pan over medium-high heat for 3-5 minutes until a drop of water sizzles immediately. Add 1 teaspoon neutral oil and swirl to coat the pan.

Pat beef slices dry with paper towels to remove excess moisture. Add beef in a single layer with 1-inch gaps between pieces, working in 2-3 batches depending on pan size. Cook for 2 minutes without moving to allow edges to brown and caramelize.

Flip beef slices with tongs or chopsticks and cook for 1 more minute until no pink remains but meat still looks juicy. Add green onions in the last 30 seconds and toss briefly. Transfer cooked beef to a plate and repeat with remaining batches.

Let cooked beef rest off heat for 2 minutes to allow juices to redistribute. Serve over steamed white rice with kimchi, pickled radishes, or lettuce leaves for wraps. Drizzle pan juices over the top if desired.