Rinse flanken short ribs under cold water to remove bone fragments, pat completely dry with paper towels, and cut between bone sections if longer than 5-6 inches.
In a medium bowl, combine soy sauce, honey, sesame oil, minced garlic, grated ginger, grated Asian pear, rice wine, black pepper, and chopped green onions. Whisk until honey dissolves completely.
Pour marinade over ribs, add sliced onion, and toss to coat evenly. Refrigerate for at least 2 hours, preferably overnight for best flavor. Flip bag halfway through marinating.
Remove ribs from marinade, reserving ½ cup for glaze. Bring reserved marinade to a boil and simmer 5-7 minutes until reduced by half and thickened.
Preheat grill to medium-high (400°F). Place ribs meat-side down in a single layer and cook undisturbed for 3-4 minutes. Flip and cook another 3-4 minutes until caramelized and internal temperature reaches 145°F.
Heat a cast iron skillet over medium-high heat. Cook ribs in batches, 2-3 minutes per side until deeply browned and crispy at the edges.
Arrange cooked ribs on a serving platter, sprinkle with sesame seeds and extra green onions, and drizzle with reduced marinade. Serve immediately while hot.