Place the thinly sliced beef in a large mixing bowl or a resealable plastic bag.
In a separate small bowl, whisk together the soy sauce, brown sugar, sesame oil, minced garlic, grated ginger, and grated Asian pear to create the marinade.
Pour the marinade over the beef, add half of the chopped green onions, and mix thoroughly so all beef slices are evenly coated. Marinate in the refrigerator for at least 30 minutes, or up to overnight for deeper flavor.
Heat a large skillet or wok over medium-high heat until very hot. Add a light drizzle of cooking oil if your pan is not non-stick.
Add the marinated beef in a single layer, working in batches if necessary to avoid overcrowding the pan. Cook for 2 to 3 minutes until caramelized and cooked through.
Serve immediately over steamed white rice. Garnish with the remaining green onions and toasted sesame seeds, and serve alongside kimchi, seasoned spinach, and pickled radish.