Prepare the Beef
Slice beef very thinly against the grain. Pat dry with paper towels.
Make the Marinade
In a bowl, whisk together soy sauce, brown sugar, sesame oil, garlic, grated pear or apple, and black pepper.
Marinate
Add beef to the marinade and toss until fully coated. Cover and refrigerate for at least 30 minutes.
Heat the Pan
Heat a large skillet or grill pan over high heat. Add oil if needed.
Cook the Beef
Add marinated beef in a single layer. Cook quickly, stirring occasionally, until browned, caramelized, and cooked through.
Finish
Add sliced onion and green onions during the last 2 minutes of cooking. Remove from heat and serve immediately.